74 ON THE NAIADES, 



Shell elliptical, compressed, inequilateral ; valves rather thin ; beaks compressed and 

 undulated at tip ; epidermis yellow with interrupted dark green rays ; cardinal teeth 

 small ; lateral teeth long and slightly curved ; nacre pearly white and iridescent. 



Hab. Harpeth River, Tennessee. Professor Troost. 



My Cabinet. 



Cabinet of Professor Troost. 



Diam. -S, Length 1-6, Breadth 2-6 inches. 



Shell elliptical, compressed, inequilateral; substance of the shell 

 rather thin, thicker before ; beaks compressed and finely undulated at 

 the tip ; ligament short and rather thick ; epidermis fine yellow with 

 numerous oblique interrupted rays, which are strongly pencilled at the 

 commencement of each stage of growth ; cardinal teeth very small and 

 erect ; lateral teeth long and slightly curved, in the left valve enlarged 

 near the termination ; anterior cicatrices distinct ; posterior cicatrices 

 confluent ; dorsal cicatrices in the centre of the cavity of the beaks, and 

 deeply impressed ; cavity of the beaks very shallow and rounded ; nacre 

 pearly white and iridescent. 



Remarks. — This species, so beautiful and so peculiar in its painted 

 exterior, 1 owe to the kindness of professor Troost. The fine specimen 

 figured belongs to the museum of that gentleman in Nashville, and I 

 am indebted to him for the loan of it to insert it here. It belongs to 

 a group, the peculiar character of which seems to be in the singular 

 interruption of the rays, which are obsolete, except at the commence- 

 ment of each stage of growth, where they are strongly pencilled with 

 green. The U. plamilatus (nobis), U. patulus (nobis) and U. perdix 

 (herein described) belong to this group. The U. pictus has some re- 

 semblance to the U. cariosus (Say), but differs in being more com- 

 pressed, and in having rays over the whole disk. It perhaps more 

 closely resembles the younger specimens of U. crasstis (Say). It dif- 

 fers, however, in being thinner, smaller, and in the character of the 

 rays. 



