454 NEW GENL'S AND SOME NEW SPECIES 



Diam. -6, Length 2-2, Breadth 2-5 inches. 



Shell narrow-elliptical, inequilateral, transversely wrinkled, 

 much compressed ; substance of the shell very thin ; valves con- 

 nate over the ligament, and not elevated into a wing ; beaks 

 scarcely prominent, pointed, nearly terminal ; epidermis wrin- 

 kled, yellow, with very oblique green rays, which, when ap- 

 parent, give a greenish hue to the shell ; rays more numerous 

 and perceptible along the umbonial slope ; marks of growth 

 very perceptible ; greatest diameter along the umbonial slope : 

 cardinal tooth of right valve a small lobe closing into a de- 

 pression of the margin of the left valve ; lateral teeth acicu- 

 lar, single in both valves, and nearly or quite direct ; anterior 

 cicatrices distinct ; posterior cicatrices confluent ; dorsal cica- 

 trices form a line across the cavity of the beaks, and are very 

 perceptible ; cavity of the beaks scarcely apparent ; nacre 

 bluish-white, purple about the region of the teeth and the 

 cavity of the beaks. 



Remarks. — This interesting species is the most fragile and 

 thin of all the family of the Naiades which I have seen. The 

 epidermis seems in some specimens to prevail over the sub- 

 stance of the shell, which is so extremely brittle as almost to 

 be destroyed in our cabinets by its contraction from the effect 

 of the atmosphere. The beaks are so nearly terminal that it 

 somewhat resembles the modiola in this respect. It is the near- 

 est approach to the Jlnodonta, having but the rudiments of 

 teeth ; and I am much disposed to believe that the " Jlnodon 

 purpurascens" of Swainson is analogous to this shell. He had 

 seen but one perfect specimen sent him by Mr Rafmesque from 

 the "back settlements." I have seen many specimens of the 

 tenuissima, all of which have the rudiments of the cardinal and 

 lateral teeth. This shell exhibits to us the necessity of resort- 

 ing to a more natural definite division of Naiades than that of 

 the teeth. The tenuissima resembles most the gracilis. They 

 differ, however, in the latter being much larger, more ovate, 

 heavier, more ventricose, and not radiate. The teeth of the 

 gracilis are well defined, which is not the case with this. 



