256 PLEUROSIGMA. 



aa. LONGITUDINAL AND TRANSVERSE STRLE EQUALLY DISTANT. 



PI. Balticum W. Sm. (S.B.D., i., p. 66, pi. 22, f. 207, pi. 23, f. 207, 

 Front, p. 207 ; H.V.H. Atl., pi. 20, f. 1 ; Type No. 180), plate 7, fig. 272. 



Valve attenuate-linear, with apices obtuse-sigmoid. Raphe sometimes a 

 little more and at others a little less sigmoid. Longitudinal and transverse 

 striae equally distant (sometimes the transverse not quite so close as the longi- 

 tudinal), about 15 in 1 c.d.m. Length, very variable, 21 to 36 c.d.m. 



Marine. Common. Blankenberghe, Ostend, Antwerp (Scheldt), and everywhere. 



var. Brebissonii (PI. Scalprum Brcb. ; H.V.H. Atl., pi. 21, f. 6*; 

 Types Nos. 188 and 189), plate 7, fig. 273. 



Much smaller and more delicate than the type form, slightly more 

 sigmoid, with a raphe dividing the valve symmetrically throughout its 

 length. Striae about 22 or 23 in 1 c.d.m. Length, 8 to 10 c.d.m. 



Marine. Ostend, Blankenberghe (basin), mixed with the type form and abundant. 

 Antwerp (Scheldt), Liverpool (Stolt.), Yorkshire (Norman), Sussex (W. Sm.). 



Prof. Cleve unites var. Brebissonii with P. acuminatum. 



var. Wansbeckii {PL Wansbeckii Donk ; PL Balticum, var. /$ 

 IV. Sm.). 



Smaller and more acute than the type form, to which it is united by all 

 possible variations, though much smaller; more finely striated, 19 or 20 

 striae in 1 c.d.m. (H. Paragallo). 



Marine. Mixed with type form. 



PI. acuminatum (Kutz.) Grun. ! nee W. Sm. (Neue o. 



ung. gek. Alg., i860, p. 561, pi. 4, f. 6 ; PL lacustre IV. Sm. ; H.V.H. Atl., 

 pi. 21, f. 12* ; Type No. 181), plate 7, fig. 274. 



Valve acute-lanceolate, conspicuously sigmoid. Longitudinal and trans- 

 verse striae 17 or 18 in 1 c.d.m. Dry valve yellowish. Length, 13 to 17 

 c.d.m. 



Fresh water. Common. 



PI. reversum Greg. (Diat. of Clyde, p. 58, pi. 6, f. 105), plate 28, 



fig. 797. 



This is a very doubtful species. Only two examples have been seen by 

 Gregory, and no one else has ever seen an example. The valve is inflated at 

 the median portion and at the apices. 



aaa. LONGITUDINAL STRL MORE APPROXIMATE THAN THE TRANSVERSE. 



a. J'alves with apices not rostrate produced. 



PI. strigilis W. Sm. (S.B.D., i., p. 61, pi. 22, f. 208; H.V.H. 

 Atl., pi. 20, f. 2* ; Type No. 176), plate 7, fig. 275. 



a 



