60 LAND AND FRESH-WATER SHELLS. 



(b) Shell brownish-green, or yellowish-horn colour, strongly 

 striated, oval, ventricose; marked sometimes with two or three 

 dark bands ; umbones blunt, central ; ligament conspicuous ; 

 muscular impressions distinct; teeth as in S. corneum. Length 

 |ths inch; width T 7 ^ths inch; breadth T %ths inch. Canals and 

 sluggish streams. S. rivicola. 



(c) Shell pale grey, compressed, oblong, faintly striated; 

 umbones small, central ; ligament long, narrow, visible externally ; 

 cardinal teeth very small ; muscular impressions distinct. Length 

 \ inch ; width f ths inch ; thickness Jth inch. Ponds and rivers. 

 Local. S. palliduiu. 



(d) Shell yellowish-white or ashy, more compressed than S. 

 corneum, subrhomboidal ; umbones narrow, central, capped with 

 the nucleus of the shell ; ligament narrow, slightly visible ; muscular 

 impressions indistinct, teeth smaller and shorter. Length |rd inch, 

 width Jth inch ; breadth ^th inch. Ditches, marshes, ponds, canals, 

 and rivers. Generally distributed, but local. S. lacustre? 



2. Pisidium. Shell inequilateral. Animal with one 

 siphon. 



(a) Shell triangular, tumid, deeply grooved concentrically, 

 whitish-grey or pale brown ; umbones blunt, rather prominent ; 

 ligament short, narrow, conspicuous ; teeth as in Sphcerium. 

 Length |rd inch ; width Jth inch ;' thickness T n ^th inch. Canals, 

 ponds, lakes, and rivers. P. amnicum? 



(b) Shell triangular, very tumid so as to be cuboid in shape, 

 greyish-white, finely striated; ligament short, scarcely visible; 

 umbones pointed, prominent; muscular impressions distinct. 

 Length Jth inch ; width tth inch. Canals, ponds, ditches, and 

 sluggish streams. P.fontinak^ 



(c) Shell oval, compressed but swollen, yellowish-white or 

 greyish-horn colour, thin, finely striated; umbones blunt, short, 



2 v. rotunda, shell rounder, flatter, yellowish-green ; v. ryckholtii, shell 

 small, globular, triangular, umbones pronounced ; v. brochoniana, shell larger, 

 flatter, umbones smaller. 



3 v. striolata, smaller, wrinkles more pronounced; v. Icevinscula, larger, 

 wrinkles demi-effaced. 



4 v. cinerea, larger, flatter, stria; fainter ; v. pulchella, more glossy, strongly 

 grooved, umbones less acute ; v. henslowana, with a plate-like appendage near 

 the umbones. 



