SYSTEMATIC ACCOUNT 271 



Cells discoid, medium to large. Valves convex, undulate. Hyaline area in the centre 

 of the valve, usually occupying one-third of the total diameter of the valve surface. 

 Hyaline rays proceed from the central area to the periphery. Rays usually seven in 

 number, mostly narrow, one narrower than the others. Peripheral sectors punctate, 

 puncta small, arranged in tangential lines. The marginal row of puncta upon the inner 

 margin of each sector is usually more distinct than the others. Apices of sectors 

 flattened, having rounded corners. Central hyaline area traversed by a number of 

 radiating branched lines, joining the apices of the sectors. Chromatophores : numerous 

 irregular bodies. Diameter of valve 100-120/x. 



A handsome species usually considered as oceanic, but found frequently around 

 South Georgia. 



Observed at Sts. 475, 478, 503, 505, 508, 509, 510, 666. 



Genus Actinoptychus Ehrenberg 

 Ehrenberg, 1843 



The type species of this genus, Actinoptychus senarius, was originally described as an 

 Actinocyclus by Ehrenberg (1838). When the latter genus was established two species 

 were described, A. senarius and A. octonarius. Recognizing that structural differences 

 existed between them, Ehrenberg separated A. senarius and made it the type of a new 

 genus Actinoptychus 1841 (1843), pi. 1, fig. 27. 



Actinoptychus senarius (Ehrenberg) Ehrenberg. 



Ehrenberg, 1843, pi. 1, part 1, fig. 27. 



Actinocyclus senarius Ehrenberg, 1838, p. 172, pi. 21, fig. 6. 



Actinocyclus undulatus Kiitzing, 1844, p. 132. 



"Actinoptychus undulatus (Bailey) Ralfs", in Hustedt, 1929, p. 475. 



"Actinoptychus undulatus (Bailey)", in Lebour, 1930, p. 51. 



Actinocyclus sp., in Bailey, 1842, p. 94, fig. 11 (unnamed). 



Cells discoid. Valves divided into mostly six sectors, alternately raised and depressed. 

 Small hexagonal central area present. Sectors covered with coarse polygonal areolation, 

 areoles large and small, irregular. Marginal areoles large, often elongated radially. 

 Secondary structure in the nature of fine puncta upon the inner surface of the areoles. 

 Each sector furnished with a stout apicule placed near the margin of the valve. Valve 

 surrounded by a narrow outer margin, finely striate, and bearing a ring of fine spinulae. 

 Chromatophores: several large plates. Diameter of valve 40-130/x. 



A very variable species, with a wide distribution in subtropical seas. It has been re- 

 ported from the Mediterranean, and occasionally from the North Sea. It was observed 

 frequently around South Africa and in the Brazil Current. 



It has been often said that there existed in the minds of early diatomists a confusion 

 between Actinoptychus and Actinocyclus. Although this has now been settled, the fol- 

 lowing notes are included in order to make clear the reasons which have led to changes 

 being made in the names of some well-known species. Ehrenberg (1838, p. 171) 

 established the genus Actinocyclus. Two species were described and figured, A. senarius 



D XVI 16 



