126 



TRIBE V. CONCHACEA. 



With at least three primary teeth in one valve, and the other 

 generally the same, although sometimes with less. 



SuB-DivisroN I. — Inhabiting the sea. Usually destitute of 

 lateral teeth. 



Ge)ius 19 — VENERICARDIA — Lamarck. 



Generic Character. — Equivalve, inequilateral, suborbicular ; 

 the sides generally with longitudinal radiated ribs ; hinge with 

 two oblique teeth in each valve, turned in the same direction. 



Venericardia imhricata. — The Imbricated VENEaiCARDiA. 

 Plate XVI. fig. 17. Suborbicular, having convex longitudinal 

 ribs, covered with imbricated, rough scales. One and a half 

 inch long. Fossil at Grignon. 



Genus 20. — ORTYGIA — Leach. 



Generic Character. — Shell equivalve, transverse, three primary 

 teeth in each valve, two of which are approximate and the other 

 remote ; with the ligament subexternal ; umbones turned much 

 to one side ; beneath which is an elongated cordiform depression ; 

 cartilage slope much depressed, with an elongated groove in the 

 right valve for the reception of the margin of the left valve. 



Orti/gia gallina.— The Hen Ortygia. Plate XV I. fig. 19. 

 Somewhat heart-shaped with obtuse recurved concentric striae, 

 and three or four radiated bands proceeding from the umbo to 

 the margin, frequently with zigzag markings throughout ; inside 

 white, the margin finely crenated. One inch long. Inhabits 

 the British seas. 



Genus 21— VENUS.— LmnrtJM.?. 



Generic Character. — Equivalve, inequilateral, transverse, or 

 suborbicular ; hinge with three teeth in both valves, all approxi- 

 mate, the lateral ones divergent at their summits ; ligament ex- 

 ternal ; having cordiform depressions beneath the beaks. 



