44 EAMBLES IN SEAECH OF SHELLS. 



in various parts of the Thames, and marshes 

 around London (PL V., figs. 5, 6, 7). Formerly 

 these were all classed with Sph(€rium (or Cyclas, as 

 it used to be called) ; but, independently of their 

 smaller size, the species of the genus Pisidium differ 

 from those of Sph(ermm in the shape of their shells, 

 which are not equilateral — that is, the beak is situated 

 near the shorter end — and in having but one tube or 

 syphon instead of two. It may here be desirable to 

 explain that in the family SphcEriidce, the mantle is 

 open in front, and forms at the posterior side a cylin- 

 der, which is often divided near its opening into two 

 contractile tubes, one for respiration and nutrition, 

 the other for excretion. The members of this 

 family, being gregarious, are often met with in 

 considerable quantities, the species of Pisidium 

 looking not unlike peas (hence their generic name) 

 scattered about. The distinguishing character of 

 the species in these two genera have puzzled many, 

 and some idea of the great variation which exists 

 may be formed from the fact that, out of forty-one 

 so-called European species of Pisidium, twenty-one, 

 according to Mr. Gwyn Jeffreys, are referable to 

 and are mere varieties of P. foniinale, and only six 



