SYSTEMATIC ACCOUNT 2 6 3 



three to 120. For each of these variants he proposed a specific name. Subsequent re- 

 search showed that this variation could not be considered a specific character, as it was 

 often found that the two valves of one frustule might possess different numbers of com- 

 partments. Ralfs (in Pritchard, 1861, p. 834) gathered all these names together under 

 " Actinocyclus Ehrenbergii new species". This procedure is illegal. In accordance with 

 the Rules of Nomenclature the name first applied to a member of a group shown to be 

 synonymous, must be the name selected. Actinocyclus octonarius Ehrenberg was in- 

 cluded by Ralfs in "Actinocyclus Ehrenbergii new species". As this is the type species, 

 it is the oldest name of any member of the group of 1 18 variants. It is here reinstated in 

 place of Ralfs 's combination. 

 Observed at Sts. 260; WS 481. 



Actinocyclus rotula Brun. 



Brun, 1891, p. 6, pi. 17, fig. 5. 



Cells discoid, small. Valves flat. Valve surface divided into a number of segments, 

 usually 12-16, by radial lines of granules. These radial lines terminate in a small spur 

 or apicule at the margin of the valve. Small central hyaline area present. The interstitial 

 segments contain a number of puncta, irregularly arranged. Puncta sparse, but usually 

 more dense at the marginal area of the valve. Margin of valve strong, radially striate. 

 Chromatophores : numerous small plates. Diameter of valve 66/u. 



Observed at St. 260. 



Actinocyclus umbonatus Castracane. 



Castracane, 1886, p. 145, pi. 4, fig. 4. 



Karsten, 1905, p. 91, pi. 9, fig. 1. 



Actinocyclus valdiviae Karsten, 1905, p. 92, pi. 9, fig. 3. 



Actinocyclus antarcticus Karsten, 1905, p. 91, pi. 9, fig. 2. 



Cells discoid, valves convex, surrounded with a broad flat margin. Margin radially 

 striate, the striae sometimes proceeding from a ring of fine puncta. Striate portion of 

 the margin often bounded by a narrow plain, or hyaline rim. Valve surface covered with 

 moniliform striae often arranged in radiate fascicules or in tangential sectors of curved 

 lines. Puncta uniform in size throughout the whole valve surface, central hyaline area 

 sometimes present, small. The marginal mark, or pseudo-ocellus is prominently placed 

 upon the striate margin. Mark elongated, rectangular, spatulate, or clavate. Marginal 

 spinulae absent. Chromatophores: numerous flattened bodies, irregular in shape, some- 

 what angular. Diameter of valve 54-138/ii. 



A neritic species, which undergoes much variation. Widely distributed throughout 

 the Southern Ocean, but seldom found in great numbers. 



From the vast amount of material I had at my disposal, I found no difficulty in 

 establishing the fact that Karsten 's species were but variants of A. umbonatus Castra- 

 cane. 



Observed at Sts. 477, 478, 664. 



D XVI I5 



