dM CYRENIDiE. 



stibaiiterior ; hinge-plate rather wide ; cardinal and lateral teeth 

 t3'"pical, excepting that the cross-strise are ver3^ obscure or obso- 

 lete and the posterior lateral rather short and very remote from 

 the cardinal, with a wide, flattened space intervening ; surface 

 concentrically striated. Gorbicula fracfa, Meek. 



Sph^rium, Scopoli, 1171. 



Syn.—Cyc]sxs, Brug., 1192. Cornea, Muhlf., 1811. Corneo- 

 cyclas, Fer., 1818. 



Didr. — 15 sp. Universal. Fossil. Laramie — ; N. America. 

 Eocene — ; Europe. S. corneum, Linn, (cxiv, 41). 



Shell thin, oval or suborbicular, inflated, covered by a greenish 

 epidermis ; cardinal teeth very small or rudimentary, one more 

 or less bifurcated, one in the right and two oblique ones in the 

 left valve ; lateral teeth compressed, lamelliform, the anterior 

 shortest ; ligament short ; margins plain, muscular impressions 

 scarcel}'^ apparent, submarginal ; pallial impression simple. 



Animal oval, subglobular ; mantle-margins plain ; siphons 

 unequal, not ciliated, short, only united at the base, the branchial 

 one largest and longest ; mouth small, oval, transverse ; branchial 

 large, unequal, united behind, the inner ones largest ; foot tongue- 

 shaped, triangular, flattened, very extensible. 



The fry of Sphserium are hatched in the internal branchis, 

 they are few in number and very unequal in size ; a full-grown 

 G. cornea has about six in each gill ; the largest being one-sixth 

 to one-fourth the length of the parent. The young Sphseria and 

 Pisidia are very active, climbing about submerged plants and 

 often suspending themselves by byssal threads ; the striated 

 gills and pulsating heart are easily seen through the shell. 



CYRENASTRUM, SPH^RIASTRUM, Bourg. ; CORNEOLA, CALYCULtNA, 



Clessin,are names given to sections of the genus, possessing but 

 slight difi'erential characters. 



PisiDiUM, Pfeiffer, 1821. 



Syn. — Pera, Leach ? Musculium, Link, 1801. Pisum, Muhlf., 

 1811. 



Distr. — 60 sp. Universal. Fossil. Laramie ; N. Am. Eocene — ; 

 Europe. P. compressum, Prime (cxiv, 48 ; cxv, 48). 



Shell suboval, trigonal, inequilateral, covered by a greenish 

 epidermis ; cardinal teeth very small, elongated, one, sometimes 

 bifurcated, in the right valve, two, diverging, in the left valve ; 

 lateral teeth longitudinal, compressed, lamelliform, double in 

 the right valve. 



Siphons short, simple, contractile, united to the end ; foot 

 linguiform, flattened, very extensible. 



This is closely related to the preceding genus ; but Pisidium 

 has siphons united to the end, short ; Sphserium, siphons elon- 

 gated, separate. In Pisidium the cardinal teeth are elongated, 



