PLEUROSIGMA. 255 



portions. Transverse stria? delicate, about 18 in i c.d.m. ; oblique striae 

 robust, 13 or 14 in 1 c.d.m., cutting one another at nearly a right angle. 

 Length, about 25 to 30 c.d.m. 



Marine Washings of mussels ; more frequent than the preceding, of which, according to Mr. 

 Kitton, it is only a variety. Same localities. 



II. Stria rectangular, cutting one another in only two directions ; stria 



longitudinal and transverse. 



* Frustules not enclosed in a gelatinous tube. 



a. LONGITUDINAL STRIDE MORE DISTANT (AND CONSEQUENTLY MORE 

 DISTINCT) THAN THE TRANSVERSE STRLE. 



PI. Hippocampus W. Sm. (S.B.D., i., p. 68, pi. 22, f. 215; 

 H.V.H. Atl., pi. 20, f. 3*; Type No. 179), plate 7, fig. 270. 



Valve short, rather broadly lanceolrte, with apices obtuse, abruptly sigmoid ; 

 longitudinal striae, 10 or n in i c.d.m., transverse striae, 15 or 16 in 1 c.d.m., 

 median striae slightly radiate. Length, 13 to 16 c.d.m. 



Brackish water. Antwerp (Scheldt), Blankenberghe. England (Kitton, Comber, Norm., 

 Stolt.). Ireland (OMeara). On all our coasts. 



PI. attenuatum W. Sm. (S.B.D., i., p. 68, pi. 22, f. 216 ; H.V.H. 



Atl., pi. 21, f. 11* ; Type No. 182), plate 7, fig. 271. 



Only differs from the preceding by its form being slightly thinner (Grunow) 

 and its fresh water habitat. Length, 19 to 25 c.d.m. 



Fresh water. Common and general. 



Prof. Cleve unites PI. attenuatum IV. Sm. with PI. Hippocampus IV. Sm. 



var. scalprum (Gaill.) Grun. (Arct. D., p. 55 ; PI. acuminatum 

 Sm. nun Grun. ; H.V.H. Atl., pi. 20, f. 4*), plate 7, fig. 285. 



Slightly broader and not quite so long in proportion as the preceding, 

 and the striae a little finer. Longitudinal 14 or 15, and transverse striae 

 17 or 18 in 1 c.d.m. 



PI. littorale W. Sm. (Ann. N.H., 1852, p. 10, pi. 2, f. 8 ; 

 S.B.D., i., p. 67, pi. 22, f. 214*), plate 28, fig. 796. 



Smaller and broader at the median portion, but nevertheless quite distinct, 

 in consequence of the longitudinal striae being very robust and distant, 7 to 9 

 in 1 c.d.m ; transverse striae, 15*5 to 17 in 1 c.d.m 



Marine. Coasts of England and France. 



