264 PISIDIAD^. 



gills two on each side ; shell equlvalve ; cardinal 

 teeth two, diverging ; lateral teeth distinct. 



Fig. 64. 



Pisidium amnicum. — c. Foot ; d. Single siphon. 



1. Pisidium. (Pera.) 



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 Pisi- 

 dium. Leach long ago separated it in his MSS. 

 under the name of Pera, and afterwards of Englesia ; 

 and the shells, with his names attached to them, were 

 long exhibited in the British Museum collection, 

 before they were described by Mr. Jenyns. 



a. Shell very slightly inequilateral, nearly rounded. 



120. 1. Pisidium ohtusale. Gibbous Pera. — Shell 

 globose, obliquely subovate, shining, very 

 finely striated, greenish black, with a yellowish 

 marginal zone, rarely all yellowish ; umbones 

 rather prominent, very blunt, (t. 12. f. 149.) 



Cyclas gibba. Alder., Cat. 41. — Telllna minima. Stader, 

 Coxes Trav. iii. 439. — Cyclas minima. Sttider, Verz. 93. 

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



