144 DIMYARIA. 



bones turned much to one side ; beneath which is an elon- 

 gated cordiform depression ; cartilage slope much depressed, 

 with an elongated groove in the right valve for the recep- 

 tion of the margin of the left valve. 



Ortygia Gallina The Hen Orttgia. Plate XVI. 



fig. 19. Somewhat heart-shaped, with obtuse recurved con- 

 centric striffi, and three or four radiated bands proceeding 

 from the umbo to the margin, frequently with zigzag mark- 

 ings throughout ; inside white, the margin finely crenated. 

 One inch long. Inhabits the British seas. 



Genus 21 VENUS.— Ziwwcems. 



Generic Character Equivalve, inequilateral, transverse, 



or suborbicular ; hinge with three teeth in both valves, all 

 approximate, the lateral ones divergent at their summits ; 

 ligament external ; a cordiform depression beneath the 

 beaks. 



Two sections.—* Inner margin of the valves crenulated or denticu- 

 lated. — ** Inner margin of the valves entire. 



Venus Casina — The Casina Venus. Plate XVI. fig. 

 18. Suborbicular, with transverse acute recurved ridges, 

 crenulated on the hind margin ; the depression subeordate. 

 Two inches long. Inhabits the British seas. 



Genus 22.— CYTHEREA.— Zamarcy^. 



Generic Character. — Equivalve, inequilateral, suborbi- 

 cular, trigonal, or 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 somewhat remote groove parallel to 

 the margin; without lateral teeth. 



A principal mark of distinction, betwixt the genus Yenus 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. 



