NIT2SCHIA. 395 



Valve lanceolate, with apices very slightly rostrate. Keel slightly eccentric, 

 with 6 to 8 dots in i c.d.m. Striae excessively faint (about 14 in 1 c.d.m. 

 according to Mr. Kitton). Length, 2 to 3*5 c.d.m. 



Fresh and brackish water. Antwerp (H.V.H.) Schaerbeeck (Delogne). Aid. Lieg. (De 

 Wild.). Here and there in Europe. 



var. media. (H.V.H. Atl., pi. 63, f. 2 and 3* ; Type No. 190), plate 

 16, fig. 526. 



Valves larger, with apices sometimes subrostrate, capitate, keel some- 

 what more eccentric, with 6 or 7 dots in 1 c.d.m. Length, 4-5 to 7 

 c.d.m. 



var, acuta. (H.V.H. Atl., pi. 63, f. 4*), plate 16, fig. 527. 



Valves longer, attaining to io - 5 c.d.m., with keel quite central, 6 or 7 

 dots in 1 c.d.m. 



Group 16. SlGMOIDEzE. 



Valves without sulci, keel quite central, without auxiliary lines, 

 carinal dots not elongated. Girdle face sigmoid, with apices not 

 produced. 



ANALYSIS OF SPECIES. 



f Strire very robust; carinal dots round. Brackish water species . N. Brebissonii. 



Strix' fine ; carinal f Striae fine ; size very considerable ; carinal dots 4 

 -J dots somewhat to 7 in 1 c.d.m. . N. sigmoidea. 



elongated. -J 

 Fresh water Striae very fine ; size medium ; 7 to 9 dots in 1 



species. (. c.d.m. . . . . N. vermicularis. 



N. sigmoidea (Ehr.) W. Sm. (s.b.d., i., p. 38, pi. 13, f. 

 104 ; H.V.H. Atl., pi. 63, f. 5-7* ; Type No. 392), plate 16, fig. 528. 



Valves linear, with cuneate apices. Keel central, with 5 to 7 dots in 1 

 c.d.m. Strice fine, 23*5 to 26 in 1 c.d.m. Girdle face narrow, sigmoid, with 

 truncate apices, connecting zone finely striated. Length, attaining 48 c.d.m. 



Fresh water. Rather common everywhere. 



N. vermicularis (Kutz.) Grun. ! (Arct. Diat., p. 91; H.V.H. 



Atl., pi. 64, f. t and 2* ; in Type No. 96), plate 16, fig. 529. 



Differs from the preceding species by its smaller and narrower size, its 

 closer carinal dots, 6 to 9 in 1 c.d.m., and shorter at the margin of the valve, 

 as well as by its finer striae, 32 to 34 in 1 c.d.m. Length, 9 to 22 c.d.m. ; 

 breadth of valve, about '5 c.d.m. ; breadth of girdle face, "5 to i*o c.d.m. 



Freshwater. Belgium? England (H.V.H. Type No. 96). Scotland (W. Am.), (var. lampro 

 campa. , Aid. Lieg. (De Wild. ). 



