100 DIMYARIA. CYTHEREA. 
Section 5. In the right valve the posterior teeth are very large and thick, 
and the two anterior teeth small and contiguous, the intervening space only 
fitted to receive the thin lamellar anterior tooth of the left valve; central 
tooth of the left valve thick and double, and the posterior elongated, and nar- 
row, somewhat lamellose, muricated or externally cancellated; siphonal im- 
pression large and acuminated. 
SUB-DIVISION II. — Lunule not circumscribed by an impressed line. 
Section 6. External form nearly orbicular; three divergent teeth in the 
hinge ; pallial sinus large and acute at its anterior end, with a flattened space 
under the fulerum, to which the ligament is attached: surface almost smooth. 
Section 7. External form ovate, and very thin; right valve, with the two 
anterior teeth thin, lamellar and contiguous, the intervening cavity receiving 
a thin tooth of the opposite valve, the posterior tooth double, thin, and elon- 
gated; left valve witha thick, and large central tooth, and having a very thin 
and linear posterior tobth; siphonal impression large. 
Section 8. External form approaching the Cytherez, surface smooth ; 
pallial impression with a small rounded sinus; right valve with three teeth, 
the posterior two large, and the anterior very small; left valve with three 
large teeth 
The Veneres are marine shells, inhabiting the shores of almost all countries, 
and bury themselves in the sand. 
Fossil shells of this genus are found in the beds of the Tertiary formation. 
Genus XX VI. —CYTHEREA., — Lamarck. 
Generic Character. — Shell bivalve, equivalve, generally 
more or less equilateral, or obtusely trigonal and transverse, 
or ovate ; smooth, or variously striated ; with three or more 
short divergent cardinal teeth ; and one anterior approxi- 
maie lateral tooth in both valves, situate near the primary 
teeth ; two remote, lateral muscular impressions, united by 
a pallial impression ; ligament external. 
Cytherea polita. Plate X. fig. 15. 
The shells of this genus are distinguished from those of Venus, by their 
lateral teeth. 
This comprehensive genus may be properly separated into the following 
sections. 
Section 1. Agreeing in general character with the shells of the genus Venus, 
the only difference being in the species of this section having a distinct, 
blunted, lateral tooth, and in the pallial impression being destitute of a sinus; 
most of the species are smooth on the external surface; some few have diver- 
gent striae and ribs; and others are concentrically striated; with their beaks 
inclined forwards; lunule more distinctly marked than in their congeners. 
C. scripta, and ornata, &c. illustrate this section. 
Section 2. External surface smooth, and provided with a thin corneous 
epidermis ; three divergent cardinal teeth in each valve, and a lateral anterior 
tooth situate under the elongated, and indistinct lunule; muscular impression 
of the mantle provided with a small sinus; general shape obtusely trigonal ; 
anterior side shortest ; umbones with a slight inclination forwards. C. dusoria, 
and petechialis, illustrate this section. 
This may be considered the typical group of the Cytherezx. 
