PISIDIUM. 15 



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. I. Brochoniana. Much larger and natter. 



Var. II. rotunda. Rounder and flatter. 



Var. III. Ryckholtii. Small, triangular, and globular ; 

 beaks prominent. 



Genus II. PISIDIUM. 



These little shells 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 follows : 



