VENERIDZ. 175 
and ribbed, distinctly gaping posteriorly; hinge in the right 
valve with two cardinal teeth,and a third very small, but usually 
obsolete, anterior; the middle one is prominent, curved as in 
Petricola; the posterior is longitudinally lamellar, low and 
bifurcate ; in the left valve are three distant and very unequal 
cardinal teeth; the middle one is similarly projecting as the 
corresponding tooth in the other valve. Rup. lamellifera, 
Conrad, may be considered as a type of the group. 
Croristopon (Jonas, 1844), H. and Adams, 1857. 
Syn.—Naranio, Gray, 1853. 
Distr.—3 sp. Polynesia, Mazatlan. C. divaricatum, Chemn. 
(exii, 94). 
Shell oval-quadrangular, rugose or tuberculose, swollen, beaks 
anterior; two cardinal teeth in each valve, the superior one of 
the right valve compressed and elongated, that of the left valve 
oblique and bifid; no lateral teeth; ligament external, short, 
in a deep groove ; anterior muscular impression oblong ¢, poste- 
rior one large, rounded ; pallial sinus rounded, deep. 
Saxipomus, Conrad, 1837. 
Distr.—Californian Province. S. Nuttallii, Conrad (ecxii, 97). 
Shell transversely oval, inequilateral, solid, ventricose, with- 
out lunule, umbones tumid ; teeth three or four, unequal, narrow, 
the central bifid; ligament very thick, elongated ; muscular 
impressions oval or rounded, nearly equal; pallial sinus large, 
elongated, horizontal. 
Famity VENERID®. 
Shell regular, closed, suborbicular, or oblong; ligament 
external; hinge with usually three diverging teeth in each valve; 
muscular impressions oval, polished; pallial line sinuated. 
Animal free, locomotive, rarely byssiferous or burrowing; 
mantle with a rather large ‘anterior opening; siphons unequal, 
united more or less; foot linguiform, compressed, sometimes 
grooved ; palpi moderate, tri iangular, pointed ; branchia large, 
subquadrate, united posteriorly. 
The shells .of this tribe are remarkable for the elegance of their 
forms and colors; they are frequently ornamented with chevron- 
shaped lines. Their texture is very hard, all traces of structure 
being usually obliterated. The Veneridie appeared first in the 
Oolitic period, and have attained their greatest development at 
the present time; they are found in all seas, but most abund- 
antly in the tropics. 
SupraAMity VE NERIN. 
Shell oval or subtrigonal. Siphons free to their extremity ; 
foot lanceolate, without byssiferous groove. 
