282 CONCHIFERA. 



the bottom of the vessel with the posterior extremity 

 of the shell elevated, and the siphons exserted. 



The cup, or swollen part of the umbo, in the ven- 

 tricose shell of the young animal remaining on the 

 top of the shell, is common to several species of the 

 family, and which, like many other viviparous Mol- 

 lusca, produce their young of a large size, compared 

 to their parent. 



2. Pisidium. 



Mantle extended behind into a short, simple, con- 

 tractile siphon; foot tongue-shaped, very exten- 

 sile ; shell suboval, wedge-shaped, inequilateral ; 

 hinge teeth and lateral teeth like Cyclas. 

 This genus was first distinguished by Scopoli : it has 

 since been established, from characters drawn from 

 the animal by Pfeiffer, under the name of Pisidium. 

 Leach long ago separated it in his MSS. under the 

 name of Pera, and afterwards of Euglesia, and the 

 shells, with his names attached to them,^vere long ex- 

 hibited in the British Museum collection. 



* Shell slightly inequilateral. 



115. 1. Pisidium obtusale. Gibbous Pera.(t.l2. f.149.) 

 Shell globose, obliquely subovate, shining, very 

 finely striated, greenish black, with a yellowish 

 marginal zone, rarely all 3 T ellowish; umbones 

 rather prominent, very blunt. 



Pisidium obtusale. Pfeiffer, 125. t. 5. f. 21, 22. ; 

 Jenyns, 1. c. 13. t. 20. f. 1. 3. 



Cyclas obtusalis. Lam. Hist. v. 559. 



Pera gibba. Leach, MSS. Brit. Mus. 



Young. The yellow zone broader. 



