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A reappraisal of the diatom genusRhoiconeis and the description ofCampylopyxis, gen. nov.Linda K. Medlin a ba Department of Biology , Texas A & M University , College Station,Texas, 77843, USAb Department of Botany , University of Bristol , Bristol, BS8 1UG, UKPublished online: 17 Feb 2007.

To cite this article: Linda K. Medlin (1985) A reappraisal of the diatom genus Rhoiconeis and thedescription of Campylopyxis, gen. nov., British Phycological Journal, 20:4, 313-328

To link to this article: http://dx.doi.org/10.1080/00071618500650321

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Br. phycol. J. 20:313 328 1 December 1985

A Reappraisal of the Diatom Genus Rhoiconeis and the Description of Campylopyxis, gen. nov.

By LINDA K. MEDLIN

Texas A & M University, Department of Biology, College Station, Texas 77843, USA*

The diatom genus Rhoiconeis as originally described by Grunow (1863) contains unrelated and morphologically distinct taxa. Cleve (1894 1895) included the taxa placed in Rhoiconeis in various sections of the genus Navicula. However, a study of type material of two species, Rhoiconeis bolleana and Rhoiconeis garkeana, originally placed in Rhoiconeis has shown that these taxa are not related to Navieula either. It is proposed that the genus Rhoiconeis be re-erected to include Rhoiconeis bolleana and Rhoiconeis (=Navicula) sponsalia and that a new genus, Campylopyxis, be erected to contain Rhoiconeis garkeana. Other taxa originally placed in Rhoiconeis have previously been moved to their appropriate genera. Rhoiconeis and Campylopyxis can be separated primarily by the type of striae and the cingulum.

In 1863, Grunow described the pennate genus Rhoiconeis to include species that were flexed in girdle view but could be separated from Achnanthes by the presence of a central nodule on both valves. The genus included two new species (Rhoiconeis bolleana and Rhoiconeis garkeana) and two new combina- tions (Rhoiconeis genuflexa = Navicula genuflexa Kfitz. and Rhoiconeis trinodis = Navicula trinodis Wm. Sm.). Grunow (ex van Heurck, 1880) later placed Rhoiconeis trinodis in Achnanthes, although Arnott (ex Pritchard, 1861) had earlier referred this taxon to Achnanthidium. Rhoiconeis was regarded by Cleve (1894-1895) as a hetero- genic collection of taxa, and he moved the remaining species into different sections of the genus Navicula. Recently a light micro- scope study based on three of the original Rhoiconeis species (Medlin & Fryxell, 1984a) confirmed that although these species were morphologically distinct, their identification had been confused. One of these taxa, Rhoiconeis genuflexa, was shown to be an earlier synonym of Rhoicosphenia adolfi M. Schmidt, and hence that taxon was

* Present address: University of Bristol, Department of Botany, Bristol BS8 lUG, UK.

0007 1617/85/040313 + 16 $03.00/0

moved to the genus Rhoicosphenia. Medlin & Fryxell (1984a) also showed that Rhoiconeis bolleana was closely related to Navicula sponsalia Gift. and suggested that they form a distinct genus for which the name Rhoiconeis (with an emended description) could be used, once Rhoiconeis garkeana and Rhoiconeis genuflexa had been removed. Type material of Rhoiconeis bolleana, Rhoiconeis garkeana and Navicula sponsalia has been studied, and this paper presents the nomenclatural changes and relationships involved.

MATERIALS AND M E T H O D S

Epiphytic material from Japan containing Navicula sponsalia and Rhoiconeis garkeana was provided by Dr H. Takano. Material of Navicula sponsalia epiphytic on Codium was obtained from Alexandria, Egypt. Permanent mounts and stubs for scanning electron microscopy (SEM) were prepared as previously described (Medlin & Fryxell, 1984b). Light microscopic observations were made on permanent mounts, and micro- graphs taken with a Zeiss Standard 16 micro- scope equipped with Nomarski differential inter- ference contrast optics. For SEM, specimens were examined with a JEOLCO JSM-35 and 25 SEM at Texas A&M University and with a Philips

© 1985 British Phycological Society

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501 SEM at the University of Bristol, at a maximum accelerating voltage of 15 KeV. The British Museum (Natural History), London, UK, the Naturhistorisches Museum, Vienna, Austria, and the National Institute of Water Resources of South Africa kindly loaned slides from their collections. Terminology follows that of Anonymous (1975), von Stosch (1975) and Ross et al., 1979. I have retained the term "interstriae" rather than use the term "virgae" as proposed by Cox & Ross (1980) because in some taxa the interstriae are well developed into ribs (i.e. virgae), while in others the interstriae are nothing more than the basal siliceous layer.

OBSERVATIONS

Rhoiconeis Grunow 1863 Emended diagnosis: cells flexed in girdle

view, isopolar in both valve and girdle views. Frustules heterovalvate with respect to central area. Lineolate striae with the interstriae united internally by a siliceous sheet along the valve margin thus closing off the portion of the striae along the mantle edge from the cell interior. Raphe fissure interrupted externally by a central nodule but continuous internally and accompanied by an axial costa along its entire length. Helictoglossum joins the siliceous sheet at the apices. Siliceous sheet incomplete just above the helictoglossum leaving a small hole leading into a chamber that opens to the outside by slits. C~ngulum hyaline consisting of several elements. Valvocopula composed of two segments, two pleurae.

Type species: Rhoiconeis bolleana Grunow 1863, p. 147, pl. 13, fig. l la , b.

Type locality: N. Kamschatk, Russia. Holotype: slide 131a, b Grunow Collec-

tion Naturhistorisches Museum, Wien. Material: Grunow Collection Natur-

historisches Museum, Wien: slides 131a, b - - Kamschatk; slides 1988a, b- -Matotschin Sch~irr, Kara Sea Exp. 1876; slide 1624-F. Hauck 107, Gr6nland.

Rhoiconeis bolleana Grun. The type species, R. bolleana, was examined using light microscopy alone, and much of the inter- pretation of the light microscope observa- tions (especially of the girdle region of R. bolleana) is dependent on SEM observations

of the related "Navicula sponsalia" (see below).

The cells are flexed in girdle view and isopolar in both valve and girdle views (Figs 1-3, 5-8). Both valves are linear-elliptical in outline and 40-60~m long and 9-10~tm wide in the material examined, while Cleve (1894-95) reports valves 45-95 gm long and 10-11 gm wide. The radiate striae are 8-11 in 101am at the centre becoming more numerous and more strongly radiate towards the apices. On the concave valve, three to five central striae are shorter on either side of the raphe forming a quadrate central area (Figs 1,4, 5). On the convex valve, one to three striae are shorter, thus forming a smaller quadrate central area (Figs 2, 3, 7, 9). If only one stria is shorter, then the central area may appear elliptical (Figs 3, 7). In his notebook Grunow figures a valve view of R. bolleana from slide 131 with an elliptical central area (Fig. 25), and this figure appears in his 1863 publication. However, no specimens with an elliptical central area were located on his slide 131. Only those with several shorter, central striae were found. Each of these had a distinct quadrate central area and corresponded well with Grunow's other notebook illustrations of R. bolleana drawn from the remaining slides (Fig. 27). A second illustration of R. bolteana from slide 131 is also reproduced here (Fig. 26). In this girdle view the concave valve has four shorter striae that delimit a distinct quadrate central area, while striae of the convex valve appear to reach the central nodule. If this valve were seen in valve view it would probably be seen to possess only one short stria. However, the majority of the valves on Grunow's slides possessed distinct quadrate central areas. Thus, the original published illustration of R. bolleana with an elliptical central area is misleading.

The raphe is filiform and curves down slightly over the apices (Fig. 5). The central raphe endings are dilated and deflected slightly to one side of the central nodule (Fig. 6). A lower focus reveals the axial costa that accompanies the raphe along its entire

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FIGS 1-9. Rhoieoneis bolleana. Fig. 1. Concave valve, Grunow slide 1624. Fig. 2. Convex valve, note faint line (arrow), Grunow slide 13l. Fig. 3. Complete frustule, note pleural bands (arrow), Grunow slide 1624. Fig. 4. Concave valve with four shorter striae, note faint line (arrow), Grunow slide 131. Fig. 5. Concave valve, focus on axial costa beneath central nodule, Grunow slide 1624. Fig. 6. Concave valve with three shorter striae, Grunow slide 131. Fig. 7. Convex valve with one shorter stria, Grunow slide 1624. Fig. 8. Girdle view, note cavity (small arrow) above the helictoglossum (large arrow), Grunow slide 131. Fig. 9. Girdle view, note juncture line between segments of valvocopula (arrow), Grunow slide 1624. FIGS 10-14. Rhoiconeis sponsalia. Fig. 10. Concave valve, N I W R slide 454/9063. Fig. 11. Girdle view, N I W R slide 454/9063. Fig. 12. Concave valve, note two faint lines (arrows), Manozuru, Japan. Fig. 13. Concave valve, focus on axial costa, Manozuru, Japan. Fig. 14. Concave valve with evenly shorter striae, Alexandria, Egypt. Scale = 10 gm.

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FIGS 15-18. Rhoieoneis sponsalia. Fig. 15. Convex valve, Alexandria, Egypt. Fig. 16. Girdle view, focus on cavity above helictoglossum (arrow) BM CL & M611. Diat. 120. Fig. 17. Girdle view, note faint line (arrow), Manozuru, Japan. Fig. 18. Girdle view, note juncture line between segments of valvocopula (arrow), Manozuru, Japan. FIGS 19-24. Campylopyxis garkeana. Fig. 19. Concave valve, BM slide 54310. Fig. 20. Same valve focus on apices. Fig. 21. Girdle view, focus on helictoglossum (small arrows) opposing valves and on pseudoseptem of valve (large arrow) BM CI. & M611. Diat. 120. Fig. 22. Girdle view, Grunow slide 131. Fig. 23. Isolated bands, focus on pars interior, BM slide 54310. Fig. 24. Same bands, focus on perforations. Scale = I0 gm.

length (Fig. 5). The internal raphe endings, the helictoglossa, are quite large and appear in girdle view as birefringent spots, one at each end of the frustule (Fig. 8, large arrow).

The interstriae are united to form a cont inuous siliceous sheet near the valve margin. The edge or the extension of this sheet is manifested at the light microscope level as a faint line across the striae (Figs 2, 4). This faint line is similar to that seen across the striae in Pinnularia and Caloneis,

marking the alveolar opening in these two genera. The hole in this siliceous sheet (see SEM observations of the taxon below) can be seen as a cavity above the helictoglossum in girdle view of R. bolleana with the light microscope (Fig. 8, small arrow).

On all G r u n o w ' s drawings of R. bolleana and in all specimens seen, the girdle area appears hyaline, and it is difficult to determine the actual number of girdle bands from the light micrographs. The valvocopula

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Rhoiconeis and Campylopyxis 317

FlGS 25-27. Rhoicone& bolleana. Reproductions of drawings from Grunow's notebook. Fig. 25. Slide 131, valve and girdle views. Fig. 26. Slide 131, girdle view, note differences in central area on opposing valves. Fig. 27. Slide 1777, valve view.

is wide and consists of two segments equal in length. The first segment extends beneath the valve for approximately 3/4 of the valve length. The second segment fills the remaining space. The abvalvar edge of the first segment meets the advalvar edge of the second segment at an acute angle (Fig. 9). This juncture extends from a point 1/4 of the valve length from the apex to a corres- ponding point at the opposite apex. Thus, the entire valve circumference is subtended by two obliquely abutting segments. The remaining band(s) is very" narrow and appears only as a small ridge below the wide segmented bands attached to the valve (Fig. 3).

Rhoiconeis sponsalis (Griffen) comb. nov. Basionym: Navicula sponsalia Giffen,

t971, p. 8, Figs 37-39. Synonym: Rhoiconeis bolleana BM 54310

Coll. Tulk; from seaweeds, California Rhoiconeis garkeana sensu Takano 1962, no figures. Rhoiconeis garkeana sensu Frenguelli 1939, figs 16-17. Navicula genuflexa sensu Ostrup, 1913, fig. 5.

Material: Syntype NIWR 454/9063 Giffen's slide 594 from Gordon's Bay, South Africa; BM slide C1. & M611. Diat. 120; BM slide 54310 Coll. Tulk, from seaweeds,

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California; BM slide 7341 arranged slide labelled Rhoiconeis garkeana from California (see Medlin & Fryxell, 1984a, for a full description of this slide); epiphytic sample washed from Ulva at Manozuru, Japan, (see Takano, 1961); epiphytic sample from Codium at Alexandria, Egypt.

The cells are flexed in girdle view and isopolar in both valve and girdle views (Figs 10-18). Both valves are linear-elliptical with rounded obtuse apices (Figs 10, 15, 18, 19). Valves are 20-42 I-tm long and 5-7.5 gm wide. The radiate striae are finer than in the nominate form (13-15 in 10gm) and do not become denser towards the apices. Externally the puncta are small longitudinal slits, similar to those found in the Naviculae lineolatae (Figs 28, 29, 34, 35). Internally, the interstriae are raised above the valve surface giving the striae a chambered appearance (Figs. 30-32). These well- developed interstriae (hereafter termed ribs) extend to the valve margin, but the space between them is irregularly bridged by a siliceous sheet. The position of the ribs can be seen through this siliceous layer with the SEM (Fig. 31), while with the light micro- scope, the edge of this sheet appears as a faint line across the striae (Figs 12, 17). In this species the faint line is more variable in position than in the type species, often extending onto the valve face. The siliceous layer is incomplete at each apex leaving a small hole above the helictoglossum (Figs 16, 32). This hole leads into a small cavity connected to the cell exterior by several small external slits, which lie beneath the polar raphe endings (Fig. 34). Another siliceous sheet bridges the ribs near the raphe (Figs 30, 36). The extent to which this sheet develops is very variable, usually being broader on one side of the raphe than the other. A second faint line close to the raphe can also be seen with light microscopy on some specimens (Fig. 12).

The raphe system is complete on both valves. The central external raphe endings are dilated and tear-drop shaped, similar to those of Rhoicosphenia, but are deflected unilaterally (Fig. 35). The polar external

endings curve slightly at the apex, then drop straight down the mantle (Figs 28, 29, 34, 38). These endings curve in opposite directions on sibling valves and, de facto, valves of a frustule (Fig. 38). Beneath these external raphe endings lie the slits that lead into the small chamber above the helicto- glossum (Fig. 34). The internal raphe system is bordered along its entire length by an axial costa (Figs 30,36,37). The raphe fissure opens along one side of the raphe costa, opening on the costal ridge only at the apices to form the helictoglossa (Fig. 32), which fuse with the siliceous layer near the valve margin. The raphe fissure and the accompanying axial costa are continuous over the central nodule (Figs 13, 36), but incompletely formed (?) valves show that the raphe fissures end in central pores (Fig. 37).

As in the type species, the central area of concave and convex valves can differ and is a function of the number of shorter striae that delimit the central area. On the concave valve three or four striae may be irregularly shortened (Fig. 12), but they can also be evenly shortened to form a small "butterfty"-shaped central area (Fig. 14). On the concave valve the number of shorter striae is usually less, and the central area can even be slightly oval (Fig. 15).

The girdle region is composed of three hyaline bands. The valvocopula comprises two wide segments (Fig. 39). The first segment occupies 3/4 of the valve circum- ference, while the remaining space is occupied by a second smaller fragment (Fig. 40). The segments meet at an "S"-shaped juncture line. The second and third bands are very narrow, being only one- fifth the width of the segmented valvocopula (Fig. 38). They overlap each other as normal open bands, the third band having a narrow pars exterior that can be traced, beneath the second band, back to the opposite apex.

Campylopyxis gen. nov. Genus monotypicum. Derivation: Gr. campylos = bent,

crooked; pyxis = case, box, receptacle.

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FIGS 28-32. Rhoiconeis sponsalia, Manozuru, Japan. Fig. 28. External view of convex valve. (x 4,000.) Fig. 29. External view of concave valve. ( x 3,500.) Fig. 30. Internal valve view, note position of two siliceous sheets joining the interstriae (arrow). (x 3,750.) Fig. 31. Detail of internal openings of areolae, note impression of interstriae beneath siliceous sheet. (x 8,200.) Fig. 32. Hole in siliceous sheet above helictoglossum. (x 8,200.) Fig. 33. Trachyneis aspera (Ehr.) Cleve--hole in siliceous sheet above helictoglossum. ( x 6,300.)

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Rhoiconeis and Campylopyxis 321

Species typica: Campylopyxis garkeana (Grun.) comb. nov.

Basionym: Rhoiconeis garkeana Grun. 1863, p. 148, pl. 13, fig. 12a, b.

Holotypus: Slide 131a, b Grunow Collec- tion, Naturhistorisches Museum, Wien.

Habitat: In oceano pacifico boreali inter varias algas.

Material: Grunow slide 131a, b, Kamschatk, Russia; Grunow slide 1988a, b Kara Sea Exp. 1876; Grunow slide 1624 Gr6nland; Grunow slide 2087b, Moss Beach, California; BM slide 54310 Coll. Tulk, from seaweeds, California; BM Adams Coll., Cleve & M611er Diat. 120, Marine from California; Epiphyte samples washed from agar seaweeds at Niigata, Japan, (see Takano, 1962).

Diagnosis. Frustula aspectu cingulari flexa, profunda in piano pervalvari. Multae vittae cingulares apertae, ligulatae, distinctis cum areolis. Valvae lineares-ellipticae rotundas obtusasque apices habentes. Valvis sunt margines inspissati, parvum ad apices pseudoseptum. Area axialis angusta. Area centralis elliptica, seiuncta interdum habens puncta. Striae punctatae atque ubique radiatae, quae versus apices fiunt plures; oppositae ad nodulum centralem leviter latius dispositae. Frustula heterovalvata. Valva concava perfectum habens raphium systema: raphium centralium extremitates externae quidem lacrimiformes, internae autem unciformes. Raphium polarium extremitates externae fissuram terminalem formant quae super apicem valvae in pallium extendit, internae parva formant helicto- glossa quae paene ad apices valvae extendunt. Valva convexa deminutum habens raphium systema: raphium centralium extremitates externae quidem lacrimiformes, internae autem unciformes. Raphium polarium extremitates externae ad apices valvae terminant, neque habent fissuram terminalem; internae parva formant helictoglossa quae vix ad apicem valvae sunt posita.

Frustule flexed in girdle view. Frustule deep in the pervalvar plane. Numerous

open, ligulate girdle bands with distinct areolae. Valves linear-elliptical with rounded obtuse apices. Valves with thickened margins and a small pseudoseptum at apices. Axial area narrow. Elliptical central area with occasional isolated puncta. Striae punctate and radiate throughout becoming more numerous towards the apices. Striae slightly wider apart opposite the central nodule. Frustule heterovalvate. Concave valve with a complete raphe system: external central raphe endings tear-drop shaped, internal central endings hook-shaped. External polar raphe endings with terminal fissures that extend over the valve apices onto the mantle. Internal polar raphe endings with helictoglossa that extend deep into the cell apices. Convex valve with reduced raphe system: external central raphe endings tear-drop shaped, internal central endings hook-shaped. External polar raphe endings terminate at the valve apices and lack terminal fissures. Internal polar raphe endings form small helictoglossa that are situated just short of the valve apices.

The frustules are flexed in girdle view and are isopolar in both valve and girdle views (Figs 19, 21). The cells have always been reported growing amongst other algae (Grunow, 1863, Takano, 1962), but their precise habit in relation to the macroalgae is unknown.

The valves are linear-elliptical with rounded apices (Fig. 19) and 30-47 ~tm long and 8-10 gm wide in the material examined but have been reported from 40 to 95 gm long by Cleve (1894-1895). Each valve has a small pseudoseptum at each apex (Fig. 21, large arrow). The intervening valve margin is somewhat thickened along its entire circumference (Figs 47, 48). The valve face rounds abruptly into a very deep mantle (Figs 43, 44). The narrow axial area expands

Fl~s 34-40. Rhoiconeis sponsalia, Manozuru, Japan. Fig. 34. Detail of slits beneath the terminal fissure at the valve apex. (x 13,500.) Fig. 35. Detail of central raphe endings and quadrate or butterfly-shaped central area. ( × 15,500.) Fig. 36. Detail of axial costa and internal raphe fissure passing over the central nodule. ( x 8,700.) Fig. 37. Detail of (?) incompletely formed valve showing raphe fissure leading to central raphe pores. ( × 6,500.) Fig. 38. Band detail, segmented valvocopula (S1, $2) and two pleural bands (P1, P2). ( × 6,200.) Fig. 39. Detail of segmented valvocopula, note juncture line (arrow). (× 7,800.) Fig. 40. Valve with only the large segment of valvocopula attached. ( × 7,200.)

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FlGS 41-46. Campylopyxis garkeana, Niigata, Japan. Fig. 41. External view of concave valve. ( x 1,900.) Fig. 42. External view of convex valve (same frustule). ( x 1,900.) Fig. 43. Detail of central area of concave valve (large arrow). Note isolated punctum in central nodule and interspersed short striae (small arrow) along the valve margin. ( × 8,400.) Fig. 44. Detail of central area on convex valve. ( x 8,400.) Fig. 45. Detail of terminal fissure on concave valve. ( x 11,000.) Fig. 46. Detail of convex valve lacking terminal fissure. ( x 5,700.)

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Rhoiconeis and Campylopyxis 323

into a small elliptical central area on both valves (Figs 43, 44). Isolated puncta can occasionally be found in the central area (Fig. 43), but because of the difficulty in separating valve and girdle components, I have not been able to determine if these pores continue to the inside. The distinctly punctate striae are radiate throughout, slightly more numerous at the apices (Figs 19, 41-46). Opposite the central area, the striae on both valves are wider apart, and occasionally shorter striae can be seen at the valve margins in these spaces (Fig. 43). Internally the interstriae are somewhat more strongly developed into distinct ribs. The areolae are closed internally by a velum, and the boundary of each aerola is not always distinct (Fig. 48).

The heterovalvy of the frustule is primarily a result of the concavo/convexity of the valves, but the raphe system on the two valves also differs. The concave valve is considered to possess a "complete" raphe system in which the two raphe branches are interrupted, both internally and externally, by the central nodule. The central external raphe endings are dilated to form tear- drop shaped structures (Figs 43, 44), which are not deflected to one side as in Rhoiconeis (sensu stricto). Each polar raphe ending is elongated into a terminal fissure that extends beyond the valve apex onto the mantle (Fig. 45). The fissure terminates just above several short striae and is not deflected as sometimes seen in Rhoicosphenia (Medlin & FryxeU, 1984b, figs 19, 24). The central internal endings are hook-shaped and identical to those in Rhoicosphenia (Fig. 48). The polar ending forms a very large helictoglossum, which extends well into the valve apex (Figs 21, 47). Because of the cell depth, the helictoglossum is very difficult to view with SEM (Fig. 47).

By comparison, the raphe system of the convex valve is "reduced". The internal and external central raphe endings are similar to those on the corresponding valve, but the polar external raphe endings stop at tbe valve apex and do not extend onto the valve mantle (Fig. 46). Correspondingly, the

helictoglossa are located at this point (Fig. 21), so that the opposing helictoglossa differ in position as seen in girdle view with light microscopy (Fig. 21).

This diatom can be most easily distin- guished from the preceding taxa by the presence of numerous distinctly perforated girdle bands. Each band is open, and each subsequent band opens at the opposite end of the frustule from its predecessor (Fig. 49). A complete cingulum has four bands. The first three bands are approximately the same width, while the fourth band is slightly narrower (Fig. 51). All are similarly perforated with a single row of slits along their advalvar margin. The remaining portion of each band is homogeneously silicified. The pars interior of the valvo- copula underlaps the thickened valve margin and the small apical pseudosepta (Fig. 21). The perforations of the valvocopula change from slits to round pores at the open ends of the band (Fig. 50). The second band has a tab-shaped ligula that fills the space between the ends of the valvocopula (Fig. 50). On the second band a change in pore shape also occurs towards the ligula (Fig. 49), and the small pores continue onto the ligula (Fig, 50). This pattern is repeated on subsequent bands, but the number of small pores on the ligulae decreases abvalvarly. Internally the slits open into small chambers parallel with the external slits (Fig. 52). These chambers also decrease in size, corresponding to the size decrease in the external band openings.

DISCUSSION

From the material examined it can be shown that two of the original Rhoiconeis species, R. garkeana and R. bolleana, are quite unrelated and warrant separation at the generic level. Although Cleve (1894 1895) recognized that the taxa in Rhoiconeis were unrelated, they cannot belong to the various subgenera of the genus Navicula as he believed.

Cleve placed R. bolleana in the Naviculae lineolatae. The frustule structure of the type

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FIGS 47-52. Campylopyxis garkeana, Niigata, Japan. Fig. 47. Internal view of concave valve, detail of pseudoseptum at valve apex and large helictoglossum (arrow). (× 8,200.) Fig. 48. Internal view of concave valve, detail of central raphe endings. (× 17,900.) Fig. 49. Cingulum. (× 2,500.) Fig. 50. Detail of pore structure at open ends of bands and at the flexure of the subsequent band, note pores on ligula. (× 12,000.) Fig. 51. Detail of pore structure of bands midway along one length of the frustule. ( × 12,000.) Fig. 52. Internal view of bands. ( × 3,900.)

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species of this section has been examined by Cox (1979), and her description has been useful in determining the characters R. bolleana shares with the Naviculae lineolatae. Externally the slit-like openings to the areolae are alike in both Rhoiconeis and Naviculae lineolatae. Internally the inter- striae are also strongly developed into distinct ribs in both groups.

This increased development or silicification of the interstriae into ribs appears to be the most common type of valve structure in the Pennales (Cox & Ross, 1980), and the rows of areolae lie in trough- like depressions between the interstriae. As the interstriae are not developed above the axial area and the continuous strip of silica along the valve margin, they cannot be called costae, and Cox & Ross (1980) suggest that the term interstriae be replaced with the term virgae. However, when the interstriae are themselves the basal siliceous layer, then the implication of a basket-like structure is no longer apposite.

Krammer (1982) interprets this " t rough" area between the interstriae (intercostae in his terminology) as an alveolus, while Cox & Ross (1980) would replace alveolus with the term alveolate striae and reserve alveolus for those diatoms in which the trough is not open to the interior of the frustule throughout its length but has a restricted opening, e.g. Pinnularia. However, Krammer considers that the Naviculae lineolatae and species of Cymbella, Gomphonema, Caloneis and Pinnularia all have alveolate valves. The closed alveolus, typical of Pinnularia and Caloneis, with a more or less restricted opening is not seen in any of the Naviculae lineolatae.

In Rhoiconeis the "trough"-like area described by Cox & Ross (1980) and Krammer (1982) is quite well developed, but a part is closed to the cell interior along the valve margin, and to some extent along the raphe. Following Krammer's text fig. 1 (Krammer, 1982) it seems ~hat Rhoiconeis combines aspects of the striae structure in the Naviculae lineolatae and in Pinnularia as well as Gomphoneis (Kociolek & Rosen,

1984). The multiple rows of areolae found on the external surface of Pinnularia and Gomphoneis valves are not found in Rhoiconeis, which has the slit-like openings of the Naviculae lineolatae. It appears that there may be a continuum of types of aerolar structure, from the Naviculae lineolatae to Pinnularia and Caloneis.

At first glance the raphe system of Rhoiconeis appears to be very similar to that of the Naviculae lineolatae. The presence of an axial costa along the side of the raphe tissue is common to both groups; however, the raphe fissure is continuous over the central nodule in Rhoiconeis but not in the Naviculae lineolatae. In both, the raphe fissure shifts from a lateral position to a more vertical position at the apices where the helictoglossa occur. The siliceous sheet and the cavity lying above the helicto- glossum are not found in the Navicula lineolatae. This cavity is present not only in the genus Trachyneis (Fig. 33), which has alveolate striae, but also at the foot pole of the marine diatoms Gomphonema groenlandicum Ostrup and Gomphonema kamtschaticum Grun., which have striae similar to the Naviculae lineolatae (unpublished observations). Trachyneis also has an axial costa, which attains its greatest development as a flange over the raphe fissure itself. However, the raphe fissure of Trachyneis is interrupted at the central nodule.

Externally the terminal raphe fissure in the Naviculae lineolatae is subtended by a row of pores. These pores are absent in Rhoiconeis only to be replaced by several slits. The same slits can be seen in Trachyneis, G. groenlandicum, and G. kamtschaticum (unpubl. obs.), and in each of these taxa they lead to the small chamber above the helictoglossum.

The girdle bands of Rhoiconeis are quite distinct and are completely unrelated to those found in the Naviculae lineolatae where only two unperforated girdle bands per frustule have been reported (Cox, 1979).

Trachyneis has four unperforated girdle bands (unpubl. obs.). The first two are wide,

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but the next two are narrow, as in Rhoiconeis. However, the second band occupies the space between the ends of the first band and extends down the length of the cell, although its pars exterior is very narrow. In Gomphonema groentandicum and G. kamtschaticum the girdle band arrange- ment is identical to that of Rhoiconeis (unpubl. obs.). It is very easy to see interrelationships between these taxa, and arguments for the evolution of one into the other and vice versa can be developed.

Segmented bands have also been reported in Cymbella (Krammer, 1981) in which two types of cingulum construction are present. In the leptoceros type sensu Krammer, the valvocopula is segmented into a 3/4 and a 1/4 portion, while the single pleura is segmented into two half bands. In the cocconema type sensu Krammer the valvo- copula is segmented into half bands and the pleurae into 3/4 and 1/4 segments. In each case the bands are narrow and perforated by a single row of simple pores and cannot therefore be compared to that in Rhoiconeis.

Thus, it seems that Rhoiconeis is only related superficially to the Naviculae lineolatae. Trachyneis, Gomphonema groen- landicum, and G. kamtschaticum appear to be its closest relatives among the biraphid pennate genera in terms of raphe, cingulum, and areolae structure, although all of these taxa are still within the Naviculaceae. While the areolae of Gomphonema groenlandicum and G. kamtschaticum are identical to those of Rhoiconeis, the striae in Trachyneis are almost completely closed to the cell interior, except for random isolated pores. Thus, true alveoli are better developed in Trachyneis and account for the difference in appearance of the striae of Trachyneis and Rhoiconeis seen at the light level.

Cleve (1894-1895) placed Rhoiconeis garkeana in the Naviculae microstigmaticae because of the punctate striae. Two well- known members of this section are Navicula delongnei V.H. [ = grevillii (C.A. Ag.) Heiberg] and Navicula pseudocomoides Hend. [=comoides (Dill.) H. & M. Per.]. It is useful to compare the morphology of these

two taxa with Campylopyxis. Superficially they resemble Campylopyxis garkeana, but upon closer examination, any close relation- ship must be discarded. Internal and external raphe endings, both central and polar, all differ in these species. In both N. delognei and N. pseudocomoides the central raphe endings terminate in a simple pore that is never dilated as much as those in Campylopyxis. Internally the central raphe endings are not hook-shaped, and in N. delognei the central raphe endings terminate in elongated raised structures (Cox, 1978, figs 17, 19, 21). The polar raphe endings of N. delognei and N. pseudo- comoides do not extend beyond the valve apex, and only small terminal fissures, which curve abruptly to one side are found in these taxa (Cox, 1977, 1979). Also in the Navieulae microstigmaticae, the raphe systems are the same on both valves, while in Campylopyxis the raphe systems of the two valves differ.

Although all these taxa have punctate striae, the internal structure of the areolae separates Campylopyxis from the two naviculoid species. Internal views of N. delongnei and N. pseudocomoides show that the boundary of each areola is maintained within a stria. In Campylopyxis isolated aerolae are difficult to resolve and the pore plates seem to be almost continuous. The interstriae are also more strongly developed into ribs in Campylopyxis than in the N aviculae microstigmaticae.

There are numerous girdle bands in both Campylopyxis and the Naviculae micro- stigmaticae; however, in the former taxon each is perforated by a single row of slits leading into a somewhat deeper chamber, while in the latter each band is perforated by two rows of simple pores.

Campylopyxis is not related to the Naviculae punctatae either (unpubl. obs.). In this group frustules are isovalvate and each valve possesses identical raphe systems with deeply indented terminal fissures. The helictoglossa are also small. The interstriae are not well-developed internally, and the individual areolae are distinct perforations in

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the basa l sil iceous layer with a charac ter i s t ic velum. The va lvocopu la has one row of slits, but subsequent bands have two rows of pores.

The resemblance of Campylopyxis to Rhoicosphenia, especial ly to Rhoicosphenia genuflexa (K/itz.) Medl in , is easily over- l ooked at the light microscopic level. S t ructura l ly , the striae, the centra l in te rna l and external raphe endings, and the girdle bands are ident ical under S E M ( M a n n , 1982; Medl in & Fryxel l , 1984b). The he te rova lvy of the frustule (o ther than the obvious concavo/convexi ty) in Campylopyxis is obscure at the light microscopic level, but once the differences have been seen with SEM, one can re turn to the l ight mic roscope and note the differences in he l ic toglossa pos i t ions visible in girdle view, a result of the difference in the raphe systems of the two valves. Therefore, based on the crypt ic he te rova lvy of the frustules and the overal l s imi lar i ty of the valve m o r p h o l o g y , it is believed tha t Campylopyxis is more closely re la ted to Rhoicosphenia than it is to the Naviculae microstigmaticae or the Naviculae punctatae and tha t the new genus Campylo- pyx is is justif ied. Due to this close re la t ion- ship to Rhoicosphenia, Campylopyxis should be p laced in the family Rhoicosphen iaceae . This family, or iginal ly descr ibed by Chen & Zhu (1983) (pers. comm. from D r P. Silva to D r D. G. M a n n ) has been redef ined by M a n n (1984), but fur ther e m e n d a t i o n in the family descr ip t ion is necessary to include Campylopyxis . Referring to M a n n (1984), the descr ip t ion of the frustule should be as follows: frustules he terovalvar , the concave valve having a full raphe system, the convex valve having either shor t raphe slits separa ted by a long, n a r r o w axial rib or an incomple te raphe system lacking te rmina l fissures.

These two genera, Rhoiconeis and Campylopyxis , have been r epor t ed as epiphytes , a l though no in fo rmat ion on their a t t a chmen t is avai lable . Both C. garkeana and R. bolleana are borea l in d i s t r ibu t ion , while R. sponsalia extends into more t empera te and sub- t rop ica l areas.

A C K N O W L E D G E M E N T S

I would like to thank Drs G. A. FryxeU and E. R. Cox for their support of this study and the British Museum, the Naturhistorisches Museum, Wien, and the National Institute of Water Resources in South Africa for loan of material from their collections. Special thanks are due to Dr U. Passauer for granting me permission to reproduce some of Grunow's original drawings. Prof. F. E. Round and Dr R. M. Crawford critically read the manuscript. Mr S.J . Tester provided the Latin diagnosis. Dr R. M. Crawford gave photographic assistance. The comments of an anonymous reviewer were particularly helpful.

R E F E R E N C E S

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(Accepted 21 July 1985)

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