264 
PISIDIAD^. 
gills two on each side ; shell equivalve; cardinal 
teeth two^ diverging ; lateral teeth distinct. 
Fig. 64. 
Pisidium amnicum. ■ — c, Foot ; d, Single siphon. 
1. Pisidium, (Pera.) 
Mantle extended behind into a shorty 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 Pfeilfer, under the name of Pisi- 
dium. 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, 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. — Tellina minima. Studer, 
Coxe's Trav. iii. 439. — Cyclas minima. Studer.^ Verz. 93. 
— Pisidium obtusale. Pfeiffer^ 125. t. 5. f. 21, 22. ; Jenyns, 
