398 NITZSCHIA. 



Shorter, narrower, and with apices more sigmoid and more abruptly 

 attenuate unilaterally; 7 or 8 carinal dots and 26 or 27 strise in 1 c.d.m. 

 Length, 6 to 8 c.d.m. Breadth of valves, about -75 c.d.m. 



Brackish water. Ard. Lieg. (De Wild.). 



var. nana Grun. (II.V.H. Atl, pi. 67, f. 3* ; Type No. 399), piate 

 16, fig. 539. 



Quite small, sigmoid, with 10 or 11 carinal dots and about 35 strise in 

 1 c.d.m. Length, about 4 c.d.m. 



Distinguished from analagous forms of N. Sigma by the conspicuous 

 central pseudo-nodule (Grunow). 



Brackish water. Not yet found in Belgium. 



var. brevissima Grun. (H.V.H. Atl., pi. 67, f. 4* ; Type No. 400), 

 plate 16, fig. 540. 



Valves linear, rather broad, somewhat constricted at the median 

 portion, with rostrate, gendy sigmoid apices. Carinal dots coarse, about 

 8 in 1 c.d.m. ; striae, 30 to 36 in 1 c.d.m. 



Brackish water. Piles in the Scheldt at Antwerp (H.V.H. ), Ard. Lieg. (De Wild.). 



Group 19. Spj-ctabiles. 



Valves large, slightly arcuate, with eccentric keel. Carinal dots 

 somewhat prolonged on the valve. 



ANALYSIS OF SPECIES. 



Valves large, slightly arcuate, with eccentric keel, carinal dots somewhat 

 prolonged on the valve . . . . . . N. spectabilis. 



N. Spectabilis (Ehr.) Ralfs. {Synedra spectabilis Ehr. Amer. 

 and Mikr., numerous figures; H.V.H. Atl., pi. 67, f. 8 and 9*; Type No. 

 401), plate 16, fig. 541. 



Valves linear, more or less arcuate, with apices attenuate, often rostrate- 

 capitate ; keel very eccentric, with 4 to 6 dots in 1 c.d.m., often prolonged 

 on the valve into a very short costa ; striae, ro to 12 in 1 c.d.m., distinctly 

 punctate ; length, attaining 45 c.d.m. 



Brackish water. Scheldt at Antwerp. Probably England. 



Group 20. LlNEARES. 



Valves without sulci. Keel somewhat eccentric, with round or 

 somewhat angular carinal dots, just a little elongated transversely. 

 Girdle face straight, sometimes slightly attenuate at the median 

 portion. 



