38 ON THE NAIADES, 



elevated and cleft in the left valve, single and rising from a pit in the 

 right; lateral teeth elevated, straight and lamellar: anterior cicatrices 

 distinct; posterior cicatrices confluent; dorsal cicatrices within the 

 cavity of the shell on the base of the cardinal tooth ; cavity of the 

 beaks angular and deep ; nacre pearly white and iridescent. 



Remarks. — Among the many fine shells presented to the Acade- 

 my of Natural Sciences by Mr Schoolcraft from the region of the 

 upper lakes, there was a single specimen of the present species. It 

 does not seem referable to any described species, and I have conse- 

 quently been induced to give it a separate place in the genus. It re- 

 sembles most the U. riibiginosus (nobis) in outline, but differs from it 

 in being more rounded on the inferior and posterior portions of the 

 margins, as well as in the cardinal tooth being more elevated and more 

 deeply cleft. In the rays it differs very much from that species. In 

 this specimen they are very remarkable, there being a very distinct 

 broad one anterior to the umbonial slope, covering one third of the side 

 of the disk, and two smaller, posterior to the umbonial slope. When 

 there is but a single specimen to describe from, it should be remem- 

 bered that many characters are not permanent, and I should not be 

 surprised if specimens of this species be found without a single ray, 

 although they are so striking in this. The tubercles, which are so 

 indistinct, may in other specimens be more distinct and more numerous. 

 In this case it will approach so closely to the asperrimus (nobis), that 

 it may prove to be only a variety. 



Unio geometrtcus. Plate IV. fig. 10. 



Testd trapezoidali, valde inequilaterali, transversa, compressu ; valvulis tenui- 

 bus ; natibus prominulis, rugis concentricis ; dentibiis cardinalibus in valvuld 

 utruque obliquis duplicibusque, lateralibus subrectis ; mar garitu purpurea. 



Shell trapezoidal, very inequilateral, transverse, compressed; valves thin; beaks 

 slightly prominent and concentrically wrinkled ; cardinal teeth oblique and double in 

 both valves; lateral teeth nearly straight; nacre purple. 



Hab. Bayou Teche, Louisiana. W. M. Stewart. 



