142 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



Valve and septum length, 0.133-0.219; width 0.006-0.008; width 

 at middle dilation, 0.010-0.011 mm. 



As the halves of each septum are different, one being beaded and 

 the other perforated, and as each septum alternates with the next as 

 to which end is perforated and which not, it follows as above stated, 

 that the costae in the two marginal bands of the frustule are twice 

 as many as those in the two inner bands ; for the former, representing 

 the beaded ends of all the septa, all show as costae, while the latter, 

 representing the median portions of the septa, only half show as 

 costae, the other half being perforations. 



Type.—C&t. No. 43678, U.S.N.M. 



Genus RHAPHONEIS Ehrenberg 



RHAPHONEIS AMPHICEROS Ehrenberg 



(Van Heurck, Synopsis, pi. 36, figs. 22-23; Peragallo, Diat., France, pi. 83, 

 figs. 15-18.) 



RHAPHONEIS BILINEATA Cleve 



See Dimeregramma bilineata (Cleve) Mann. 



Genus RHIZOSOLENIA (Ehrenberg) Brightwell 



As practically all the material examined for this report was dredg- 

 ings, any strictly plankton diatoms, like species of this genus and 

 of Chaetoceros, may be considered as only accidentally present. A 

 paragraph bearing on this will be found in the Introduction. 



RHIZOSOLENIA SETIGERA Brightwell 



(Micro. Journ., 1858, pi. 5, fig. 7; Peragallo, Rhizo., pi. 4, figs. 12-16.) 



Gran (Nord. Plank Diat., p. 55) claims that R. Tiebetata Bailey is 

 dimorphic, and one of his figures (fig. 67&) corresponds with the gen- 

 erally accepted idea of R. setigera, while his figure of that diatom is 

 something quite unlike (p. 53. fig. 64). But Brightwell's original 

 illustration cited above, that of Ralfs in Pritchard's Infusoria, plate 

 7, figure 31, and those of the carefully prepared monograph of Pera- 

 gallo do not correspond with Gran's distinction. I am unable to 

 throw any more light on the disagreement, but see no reason for 

 changing the generally accepted image of Brightwell's species. 



Genus RHOICOSIGMA Grunow 



As noted under Pleurosigma , this and DonMnia are with difficulty 

 separable by well-marked generic lines from Pleurosigma; but although 

 these three with Toxonidia comprise an unusually compact group, well 

 separated from other genera, and remarkably similar to each other, 

 the division into four genera on rather slight distinctions is an un- 

 doubted aid to identification. This is of course ample justification for 

 the arrangement. The distinctions between Pleurosigma and Rhoi- 

 cosigma are chiefly two : First, RJioicosigma is bent at the middle, like 



