DIMYAIRA. 127 



Venus Casino — The Monticassina Venus. Plate XVL 

 fig. 18. Suborbicular, with transversely acute recurved ridges, 

 crenulated on the hind margin ; slightly channelled behind the 

 depression. Two inches long. Inhabits the British seas. 



Genus 22.— CYTHEREA.— ZamarcA. 



Generic Character. — Equivalve, inequilateral, suborbicular, 

 trigonal, and transverse ; right valve with four primary teeth, of 

 which three are divergent and approximate at their base, and one 

 remote ; three primary divergent teeth in the opposite valve, and 

 a hollow margin parallel ; without lateral teeth. 



An undeviating mark of distinction, betwixt the genus Venus and the 

 Cytherea, is, that all the shells of the latter have four primary teeth in one 

 valve, and only three united on the other, with an isolated cavity, which is 

 oval and parallel to the margin, the lateral teeth divergent to the summit. 



Cytherea Chione. — The Chione Cytherea. Plate XVI. 

 fig. 21. Somewhat heart-shaped, strong ; covered with a 

 chestnut glossy epidermis, faintly wrinkled transversely, longi- 

 tudinally rayed ; with a cordiform depression under the beak. 

 Two and a half inches long. Inhabits the British and Medi. 

 terranean seas. 



This genus is divided into three sections. — * The anterior primary tooth, 

 with a striated cleft, or the margin dentated.— ** Anterior primary tooth 

 not striated in its cleft, nor dentated in the margin. — *** The internal mar- 

 gins crenated or dentated. 



Genus 23.— EXOLET A.— Brown. 



Generic Character — Shell strong, orbicular, beaks mu(;h 

 turned to one side, beneath which is a short cordiform impres- 

 sion ; hinge with three primary teeth, two of which are contigu- 

 ous, and the other divergent, which is broad in the right valve, 

 and cleft in the centre, to receive that of the opposite valve, 

 which is slender, cartilage situated within the outer surface of 

 the shell ; margins very broad, and somewhat concave ; edges 

 very thick. 



Exoleta orhiculata. — The Orbicular Exoleta. Plate 

 XVI. fig. 23. Orbicular, strong, with regular coarse, close set 

 transverse striae ; generally white or pale brown, with sometimes 

 three or four brown radiations emanating from the umbo, and 



