102 DIMYARIA. CYCLAS. 
external, inserted into a deep, marginal, posterior, dorsal 
sinus. 
Cyprina equalis. Plate X. fig. 14. 
The Cyprinz are marine shells; three species only are known, and they 
inhabit the Northern hemisphere. Several fossil species occur in the Tertiary 
formations. 
They may at once be distinguished from the Venus and Cytherea by their 
thick epidermis, and remote lateral teeth. 
SUBDIVISION II. —FLUVIATILE. — Shells covered with a spurious 
epidermis, and the hinge provided with lateral teeth. 
Genus XXIX.— CYRENA. — Lamarck. 
Generic Character. — Shell suborbicular, subtrigonal, 
equivalve, ventricose, inequilateral, and solid ; external 
surface covered with a strong epidermis, and the umbones 
usually decorticated ; three cardinal, and two remote lateral 
teeth in each valve; in one valve the posterior one is 
situate near the primary teeth, the anterior one being more 
remote, and placed before the ligament; in the opposite 
valve a deep groove intervenes between two teeth, one of 
which is large, and the other nearly obsolete ; two lateral 
remote muscular impressions ; pallial impression destitute of 
a sinus ; ligament external. 
Cyrena cunetformis. Plate IX. fig. 33. 
This genus is divided into two sections. 
Section 1. With the lateral teeth crenulated, striated, or serrated. 
Section 2. Lateral teeth entire, and destitute of striz, or crenulations. 
The species of this genus have a considerable similitude to those of Venus, 
Cyprina, Cytherea, and Cyclas. From the former three they are distinguished 
by their two remote lateral teeth, and by the thickness of their shell from 
Cyclas. 
The Cyrene are all fluviatile shells, and entirely inhabitants of tropical 
climates. They occur abundantly in a fossil state, in the upper Marine forma- 
tion of the Isle of Wight, at Woolwich, Hordwell, and neighbourhood of 
Paris. 
Genus XXX.— CYCLAS. — Bruguiere. 
Generic Character. — Shell generally suborbicular ; ven- 
tricose, equivalve, nearly equilateral, transverse, and thin, 
covered with a delicate olivaceous epidermis; two very 
minute, divergent, cardinal teeth in both valves, one of 
which is double in the left one; lateral teeth, two remote 
and a little elongated, laminar, compressed, and acute ; and 
four in the other, two of which are very small, situate on 
each side of the hinge; two lateral ovate muscular impres- 
