PISIDIUM. lo 
small, but prominent; ligament narrow; hinge strong ; 
teeth small; scars faint. 
In similar habitats to S. corneum, but more local 
though widely distributed; Glasgow is the only Scotch 
locality. Its prominent beaks form its characteristic 
distinction from the other three species. It is, be- 
sides, much more compressed than S. corneum, and its 
“shoulders” are sharp, like those of S. ovale, though 
less pronounced. ‘The finest specimens I possess were 
taken from the surface of the mud of a small pond 
which had been dried up for two months. All the 
specimens were alive. 
Var. [. Brochoniana. Much larger and flatter. 
Var. IL. rotunda. Rounder and flatter. 
Var. ILI. Ryckholtii. Small, triangular, and globular ; 
beaks prominent. 
Genus IT.—PISiDIUM. 
These little shélls are usually a great trouble to the 
collector on account of their variability and similarity, 
and conchologists have differed greatly as to the 
number of species. ‘The thanks of all collectors, there- 
fore, are due to Dr. Jeffreys, who has reduced the 
number to five fairly well-marked species, distinguish- 
ing them as foliows :— 
