186 CYRENIDA. 
subanterior; hinge-plate rather wide; cardinal and lateral teeth 
typical, excepting that the cross-strize are very 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. Corbicula fracta, Meek. 
Spy #riuM, Scopoli, 1777. 
Syn.—Cyclas, Brug., 1792. Cornea, Muhlf., 1811. Corneo- 
cyclas, Fer., 1818. 
Distr.—id sp. Universal. Fossil. Laramie—; N. America. 
EKocene—; Europe. S&S. corneum, Linn. (exiv, 47). 
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 
scarcely 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 Spherium are hatched in the internal branchie, 
they are few in number and very unequal in size; a full-grown 
C. cornea has about six in eaeh gill; the largest being one-sixth 
to one-fourth the length of the parent. The » young Spheria 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, SPHEHRIASTRUM, Bourg.; CORNEOLA, CALYCULINA, 
Clessin, are names given to sections of the genus, possessing but 
slight differential characters. 
Pisip1uM, Pfeiffer, 1821. 
Syn. sage Leach ? Niscanine Link, 1807. Pisum, Mebl., 
+H. /65 3 
Distr. —60 sp. Universal. Fossil. Laramie; N. Am. EHocene—; 
Europe. P. compressum, Prime (exiv, 48; exv, 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; Sphierium, siphons elon- 
gated, separate. In Pisidium the cardinal teeth are elongated, 
