PLEUROSIGMA. 257 



Valve very large, narrow, elongate, moderately sigmoid, with sub-acute 

 apices. Raphe slightly flexuous; transverse striae, 13 to 15, longitudinal 

 striae, 16 or 17 in 1 c.d.m. Length, 21 to 50 c.d.m. 



Marine and brackish water. Scheldt (H.V.H. ) Coasts of England, Ireland. Denmark and 

 probably all the Coasts of the North Sea 



PI. Spencerii W. Sm. (S.B.I)., partim.) 



Valve narrow, linear-lanceolate or lanceolate with apices more or less 

 sigmoid, obtuse or sub-acute. Transverse stria?, 18 to 22, longitudinal striae, 

 20 to 25 in 1 c.d.m. Length, 8 to 13 c.d.m. 



Fresh water or slightly brackish water. The type form and its varieties are met with every- 

 where. 



Constitutes a vast group of rather different forms, but which are united 

 together by all the intermediate forms ; the following forms may be 

 distinguished as varieties : 



var. Smithii Grun. ! {Pi. Spencerii IV. Sm. in S.B.D., i.. p. 68, pi. 

 22, f. 218: H.V.H. Atl., pi. 2t, f. 15*; Type No. 186), plate 7, fig. 

 276. 



Valves lanceolate-sigmoid. Central nodule small, elongate. Transverse 

 striae 18*5, longitudinal striae 21 '5 in 1 c.d.m. Length, 8 to 9 c.d.m. 

 Breadth, i'2 c.d.m. 



Fresh and brackish water. Antwerp; Tete de Flandre (P. Gaut.), mixed in small 

 quantities in a gathering of the var. curvula. England (Comber, Kitton, Norman), Ireland 

 (O'Mear.i). 



var. Kiitzingii Grun. ! (Neue, etc., i860; H.V.H. Atl., pi. 21, f. 

 14* ; Type No. 187), plate 7, fig. 277. 



More elongated and broader than the preceding ; central nodule 

 elongate, larger. Transverse striae 2o'5, longitudinal striae 22*5 in 1 

 c.d.m. Length, 10 to 12 c.d.m. Breadth, 1*3 to 1*5 c.d.m. 



Fresh water. Brussels (Del.). Frequent in Europe, West Indies, etc. (Grun.). 



var. acutiuscula Grun. ! (Grun. in H.V.H.'s Type No. 183). 



Form and striation as in the last, but slightly shorter, and with more 

 acute apices. Length, 8 to 10 c.d.m. Breadth, 1*2 to 1*25 c.d.m. 



Fresh water. Brussels (Del.). 



Prof. Cleve unites the two preceding varieties under the name of Pi. 

 Kiitzingii. 



var. nodifera Grun. ! (Arct. D., p. 59; H.V.H. Atl., pi. 2T, f. 13* ; 

 Type No. 184), plate 7, fig. 278. 



Central nodule elongated, with an oblique hyaline area ; longitudinal 

 striae 22 or 23, transverse striae 17 to 20 in 1 c.d.m ; the median ones 

 slightly radiate. Length, 6 to 10 cd.m. 



Freshwater. Brussels (Del.). 



s 



