A. Radial areola rows parallel to central row.
Actinocyclus actinochilus (Ehrenberg) Simonsen (Table 24) Basionym: Coscinodiscus actinochilus Ehrenberg. Synonym: Charcotia actinochila (Ehrenberg) Hustedt. References: Ehrenberg, 1844b, p. 200; Ehrenberg, 1854, Plate 35A/21, Fig. 5; Hustedt, 1958a, p. 126, Figs. 57-80; Hasle, 1968b, p. 7, Plate 9, Fig. 6, Map 2; Simonsen, 1982; Villareal 8c Fryxell, 1983, p. 461, Figs. 21-32. Large labiate processes, clearly seen with LM, positioned in an apparently hyaline band between valve areola rows and margin, at end of edge rows of fascicles. The presence and structure of a central annulus and the presence and width of hyaline spaces between radial areola rows variable. Pseudonodulus of about the same size as an areola, located in the hyaline band, evident in LM although difficult to recognize in many specimens, ranges internally (SEM) from a large circular depression to a small hole with a central depression (Villareal 8c Fryxell, 1983).
Actinocyclus circellus T. P. Watkins in Watkins 8c Fryxell (Table 24) Reference: Watkins 8c Fryxell, 1986, p. 294, Figs. 1-8. Pronounced central annulus enclosing areolae usually smaller than those on the rest of the valve and more or less linearly arranged. Areolae on the rest of the valve arranged in irregular, obscure fascicles. Labiate processes, discernible with LM, positioned at the end of the central and edge row of each fascicle. Pseudonodulus slightly away from the marginal ring of processes toward the valve center, easily discernible with LM.
Actinocyclus kutzingii (A. Schmidt) Simonsen (Table 24) Basionym: Coscinodiscus ktitzingii A. Schmidt. References: Schmidt, 1878, Plate 57, Figs. 17 and 18; Hustedt, 1930, Fig. 209; Simonsen, 1975, p. 92. Pseudonodulus, very small, obscure. Areolation fasciculated with secondary, curved rows. Marginal processes discernible with LM, positioned at the end of the central row of each fascicle. Fairly wide marginal zone, structure consisting of small areolae in two crossing systems. Small central annulus.
Actinocyclus normanii (Gregory) Hustedt f. normanii (Table 24) Basionym: Coscinodiscus normanii Gregory in Greville. References: Greville, 1859a, p. 80, Plate 6, Fig. 3; Hustedt, 1957, p. 218; Hasle, 1977, Figs. 2, 11-17, 20 and 21.
Actinocyclus normanii f. subsalsus (Juhlin-Dannfelt) Hustedt (Table 24) Basionym: Coscinodiscus subsalsus Juhlin-Dannfelt. References: Juhlin-Dannfelt, 1882, p. 47, Plate 3, Fig. 33; Hustedt, 1957, p. 219; Hasle, 1977, Figs. 1, 3-10, 15-19, 22, and 23.
Width of sectors, number of complete areola rows and relationship between number of fascicles and marginal labiate processes variable. Position of processes at the end of central rows of fascicles more stable. Processes readily discernible in water mounts. Pseudonodulus positioned in the bend between valve face and mantle and thus more difficult to discover. Central annulus indistinct or missing. Remarks: The two taxa especially differ in size and also slightly in ecology but not in morphology, and are regarded here as forms.
Actinocyclus octonarius Ehrenberg (Table 24) Synonym: Actinocyclus ehrenbergii Ralfs in Pritchard. References: Ehrenberg, 1838, p. 172, Plate 21, Fig. 7; Pritchard, 1861, p. 834; Hustedt, 1930, p. 525, Figs. 298-302; Hendey, 1964, p. 83, Plate 24, Fig. 3; Villareal 8c Fryxell, 1983, p. 453, Figs. 1-14. Areolation distinctly fasciculated giving the impression of the valve face being divided into "compartments." Fascicles separated by pronounced complete striae running from the margin to a more or less well developed central annulus. Fasciculation accentuated by hyaline areas filling out spaces left open by shorter rows adjacent to the complete striae. A wide marginal zone with areolae smaller than those on the valve face. Processes positioned at the end of edge rows and a large pseudonodulus in the bend between valve face and mantle are readily seen with LM. Remarks: Four varieties, in addition to the nominate variety dealt with by Hustedt (1930), differ in size and development of the central annulus, the amount of hyaline spaces and the width of the marginal zone. The central annulus was shown to be highly variable in cultured material of A. octonarius var. tenellus, whereas the valve areolation was stable (Villareal 8c Fryxell, 1983).
Actinocyclus sagittulus Villareal in Villareal 8c Fryxell (Table 24) Reference: Villareal 8c Fryxell, 1983, p. 458, Figs. 15-20. Areolation weakly fasciculated, hyaline spaces and a marginal zone less pronounced than in A. octonarius, otherwise similar to this species by the presence of a central annulus and in morphometric data. Marginal processes and a large pseudonodulus discernible with LM. A special feature, located close to the pseudonodulus and considered diagnostic, is evident with LM as a small heavily silicified arrowhead shaped region pointing to the valve center (Villareal 8c Fryxell, 1983).
Actinocyclus subtilis (Gregory) Ralfs in Pritchard (Table 24) Basionym: Eupodiscus subtilis Gregory.
References: Gregory, 1857, p. 501, Plate 11, Fig. 50; Pritchard, 1861, p. 835; Hustedt, 1930, p. 534, Fig. 304.
Distinct irregularly delimited central annulus filled by closely spaced areolae. Narrow, inconspicuous bundles of more or less wavy, radial areolae rows with processes at the ends of edge rows. Valve face with incomplete striae and hyaline spaces resulting in a spotted appearance, absent on valve mantle. Large pseudonodulus positioned on valve face and labiate processes on valve mantle easily discerned with LM. Remarks: The central annulus, the pseudonodulus and the densely areolated valve structure are the features emphasized in the original description of the species.
A. actinochilus—southern cold water region. A. circellus, A. sagittulus—warm-water region? A. octonarius and A. subtilis—cosmopolitan. A. normanii—brackish water, probably cosmopolitan.
A. kiitzingii—known from North Atlantic coastal waters.
B. Radial areola rows parallel to edge (side) row.
Actinocyclus curvatulus Janisch in A. Schmidt (Plate 19, Table 25) Synonyms: Coscinodiscus curvatulus var. subocellatus Grunow, Actinocyclus subocellatus (Grunow) Rattray.
References: Schmidt, 1878, Plate 57, Fig. 31; Grunow, 1884, p. 82, Plate 4, Figs. 8-16; Rattray, 1890b, p. 145; Hustedt, 1930, p. 538, Fig. 307. Areola rows slightly curved. A process at the end of each side row of the fascicles. Central annulus irregular in shape. Areolae decreasing in size close to the margin. Small irregular pseudonodulus located close to valve mantle.
Remarks: Actinocyclus curvatulus was described by means of Plate 57, Fig. 31 in Schmidt's Atlas (1878), which illustrates a part of a valve ca. 110 /xm in diameter. The prominent feature is the type of fasciculation. Coscinodiscus curvatulus Grunow (Schmidt's Atlas 1878; Plate 57, Fig. 33) with several varieties has the same fasciculation as Actinocyclus curvatulus.
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