186 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 oi Professor Troost. 



Diam. -8, 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, I 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. planulatus (nobis), U. patulus (nobis) and U. jyerdix 

 (herein described) belong to this group. The U. pidus 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. crassiis (Say). It dif- 

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

 rays. 



