3^8 NITZSCHIA. 



N. acuminata (W. Sm.) Grim. ! (Arct. Diat., p. 73; h.v.h. 



Atl., pi. 58, f. 16 and 17*; Type No. 379), plate 15, fig. 506. 



Valve broadly linear, sometimes constricted in the median portion, with 

 a very broad and conspicuous sulcus. Strife very strong, about 12-5 to 13 in 

 1 c.d.m., very feeble or absent in the sulcus. Keel without dots. Length, 

 7 to 8-5 c.d.m. 



Brackish water. Not yet found in Belgium. England (W. Sm.). Ireland (O'Meara, W.Sm.). 



Group 5. ClRCUMSUT/E. 



Valves with sulcus more or less narrow, sometimes invisible, keel 

 very eccentric, with conspicuous dots. Valves finely striate and 

 showing in addition some irregular puncta. These two kinds of 

 puncta belong to different layers of the valve. 



ANALYSIS OF SPECIES. 



Valve elliptic, very large ; string fine, undulated ; carinal dots very coarse, 



quadrangular . . , . N. circumsuta. 



N. circumsuta (Bailey) Grun. ! {Surfrella circumsuta 



Bailey; H.V.H. Atl., pi. 59, f. 8* ; Type No. 381), plate 15, fig. 507. 



Valve elliptic, very large, with sub-acute apices ; striae very fine, undulated, 

 about 26 in 1 c.d.m. Carinal dots very coarse, quadrangular, 3 to 5 in 

 1 c.d.m. ; the median somewhat distant and showing between them a vestige 

 of a nodule. Length, up to 21 c.d.m. ; breadth, up to 6*5 c.d.m. 



Brackish water. Very rare: Antwerp (P. Gaut.) Maritime Scheldt. (H.V.H. ). Flessingue 

 (Holland) (H.V.H., Deby.). England (W. Sm.). 



Group 6. Dubi/E. 



Valves analogous to those of Tryblionella but without a sulcus. 

 Keel eccentric. Girdle face attenuate at the median portion. 



ANALYSIS OF SPECIES. 



' Carinal dots elongated on the valve ; valve rather broad . . N. dubia. 



,. . , , , r Striae fine, valves small, distinctly rostrate . N. commutata. 



] Carinal dots round '. I 



valve narrow. (striae very fine, valve of medium size, sub-rostrate . N. thermalis. 



