BIVALVIA. 209 
but in general the form is transverse, with a diameter of seven to five; while in 
others there is no difference, the height being equal to the length. The hinge is 
furnished with three teeth in the right valve, and four in the left, the posterior one 
small, parallel to, and a little within the fulcrum for the ligament: in the right 
valve the posterior tooth is bifid or cleft; the surface is ornamented with numerous 
close set strie, broader than the spaces between them, and sometimes rather irregular, 
bifurcating or inosculating, more especially on the posterior part; the impression of 
the mantle is at some distance within the margin, and the sinus large, rounded, and 
reaching about one third across the shell. 
A species from the older Tertiaries at Barton, figured and described by J. Sowerby 
in ‘Min. Conch.,’ t. 422, fig. 2, (V. rotundata, Brander, figs. 91, 93,) very much 
resembles our Crag fossil, but is, I believe, distinct. 
Venus. Linnaeus. 
CaLLIsTA AND CALLISTODERMA (sp.) Poli, 1795 
ANTIGONE. Schum., 1817. 
Venuuites. Schlott., 1820. 
Craustna. Brown, 1827. 
Ortyera. ‘Leach, MS.,” id. 
TIMOCLEA. Id. id. 
Cutone. Megerle. 1811; Gray, 1851. 
Dostmna. Gray, 1838; Wood, 1840. 
Ecesta? Conrad, 1845. 
Gen. Char. Shell equivalve, more or less inequilateral, closed ; orbicular or trans- 
versely ovate, generally ornamented upon the exterior, imbricated, radiated, or 
decussated. Margin crenulated. Hinge composed of three diverging cardinal teeth 
in each valve. Ligament external. Muscular impressions roundedly ovate. Mantle 
mark sinuated. 
Animal ovate or suborbicular, with the edges of the mantle disconnected ; margins 
fringed. Siphons united at the base, separated and diverging at their extremities ; 
orifices fimbriated ; foot linguiform. 
This genus, as the name would imply, is composed of beautiful, and in the living 
state, of many highly ornamented shells, and found in the seas of all regions. 
Notwithstanding its great curtailment since the time it was established by Linnzeus, it 
now contains at least a hundred well determined species, restricting them to such as 
possess a dental armature of three diverging cardinal teeth, while the animal possesses 
two rather short siphons, giving a somewhat small and angular sinus to the impression of 
the mantle mark; the texture of the shell, at least the outer portion of it, appears to 
be of a fibrous character, and it is generally covered with fine and numerous striz in 
a radiating direction, and is more or less ornamented with reflexed portions of the 
margin or imbrications, and some few are armed with long and formidable looking 
spines. 
