248 MALACOZOA. TROPIOPODA. LAME W- IBS AN CHI ATA. 



GENUS 2. PISIDIUM. 



Animal subovate, with the mantle open anteriorly 

 and beneath, united behind into a tube, internally 

 double ; foot tongue-shaped at the end, very extensile. 



Shell subovate, subtrigonal, equivalve, very thin, con- 

 centrically striated, with a very thin persistent epider- 

 mis. Umbones tumid, approximated, little curved, 

 nearer the anterior end. Hinge with one tooth in the 

 right, and two in the left valve, besides two lateral, 

 elongated, lamelliform teeth. 



The species occur in lakes, pools, mill-ponds, brooks, 

 rivers, ditches, and marshes, generally inhabiting muddy 

 places, among the roots of plants. They are viviparous, 

 and the young, with the shell developed, attain an 

 enormous size, before they are expelled. 



The distinctive characters of Pisidium and Cyclas are 

 by many, not without reason, deemed inadequate ; the 

 Pisidia merely having the posterior syphons longer and 

 not separated at the end, and the shell generally more 

 inequilateral. The form of the shell in the same species 

 varies with age, it being more orbicular and compressed 

 when young. 



1. Pisidium Jodnnis. Johns Pisidium. 



Shell broadly ovate, well rounded at both ends, moderately 

 convex, thin, glossy, distinctly concentrically striate, the inter- 

 vening ridges broader, smooth, and polished, with more marked 

 growth-lines ; the umbones tumid, obtuse, smooth at the apex, 

 placed somewhat nearer the anterior end ; the dorsal slope a 

 little convex ; the colour greyish-yellow. Length two-twelfths 

 and a-half, height two-twelfths. 



The animal is of a greyish- white colour ; with the foot com- 

 pressed, but, when exserted, cylindrical or a little depressed, 

 obtuse, and capable of being extended to one and a-half times 

 the length of the shell ; the siphon very short, abrupt, and 

 plain on the margin. 



The shell varies little in outline, but greatly in the degree of 

 convexity, young individuals being much compressed, others 

 moderately so, and few very convex. The colour scarcely 

 varies. 



