PISIDIUM. 125 



* * Valves very inequilateral. 

 P. cinereum, Alder. 



Broadly ovate, greyish or cinereous, rather compressed, finely 

 striated ; margin of the valves meeting at rather an acute angle ; 

 umbones obtuse, and not much produced (sometimes a little 

 capped). 



Plate XXXVI. fig. 2. 



Pisidium cinereum, Alder, Suppl. Cat. L. and F. \V. Moll. Northumb. p. 4 ; 



Mag. Zool. and Bot. vol. ii. p. 119. — Gray, Manual of 



L. and F. W. Shells, p. 286, pi. 12, f. 152.— Tim.u P. 



Ann. Nat. Hist. vol. vi. p. 196 — Brown, 111. Conch. G. 



B. p. 95, pi. 39, f. 28. 

 Ci/clas cinerea, Hanley, Recent Shells, vol. i. p. 90, suppl. pi. 14, f. 44. 



This species, as Mr. Alder observes, is the largest of our 

 minute Plsidia ; and may readily be distinguished, for the 

 most part, from its congeners, by its more compressed shape 

 and ashy hue. Its form is somewhat obliquely obovate, its 

 valves are rather compressed, and its surface is covered 

 with a shining epidermis of a greyish ash-colour, which 

 sometimes becomes paler at the margin, and is somewhat 

 narrowly zoned at the stages of growth with a darker tint 

 of the same colour, and only irregularly, and occasionally 

 somewhat indistinctly, striolate in a concentric direction. 

 The posterior side is much the shorter and decidedly the 

 broader ; its width is, however, a little diminished below 

 by the greater rise of the ventral margin (which is more or 

 less arcuated) on that side. The produced anterior side is 

 moderately attenuated, but rounded at its extremity. The 

 dorsal edges are short, and their declination is very trifling ; 

 there is usually a slight angulation, especially behind, 

 where they unite with the lateral margins. The ligament 

 is very indistinct. The umbones are broad, obtuse, not 



