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 striae, 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. roiundata, Brander, figs. 91, 93,) very much 

 resembles our Crag fossil, but is, I believe, distinct. 



Venus. Linnceus. 



Callista and Callistoderma (sp.) Toll, 1795 

 Antigone. Schum., 1817. 

 Venulites. Schlott., 1820. 

 Clausina. Brown, 1827. 

 Ortygia. "Leach, MS.," id. 

 Timoclea. Id. id. 



Chione. Megerle. 1811; Gray, 1851. 

 Dosina. Gray, 1838; Wood, 1840. 

 Egesta? 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 Lmnseus, 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 striae 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. 



