60 LAND AND FRESHWATER SHELLS. 



(^) 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. contemn. Length 

 10 lines; width 7 lines; breadth 5 lines. 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 

 J inch ; width |ths inch ; thickness J inch. Ponds and rivers. 

 Local. S. ovale. 



(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 4 lines ; 

 width 3 lines ; breadth i| lines. 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 4 lines; width 3 lines; thickness i| lines. 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 2\ lines; width i| lines. Canals, ponds, ditches, and 

 sluggish streams. P.fontinale.^ 



(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. Itzviuscula, larger, 

 wrinkles demi-effaced. 



4 v. dnerea, larger, flatter, striae fainter ; v. pulchdla, more glossy, strongly 

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

 the umbones. 



