36 ON THE NAIADES, 



dus; it is, however, more oblique, and in the epidermis more shining. 

 It is not so oblique as the sulcatus, but has a furrow anterior to the 

 umbonial slope similar to that species. In the epidermis it differs very 

 much, the sulmtus being finely wrinkled and finely rayed. In some 

 specimens the successive rows of teeth along the posterior margin cause 

 that portion of the shell to swell out, which gives it a rich and beauti- 

 ful appearance. It is, though small, among the most interesting of 

 our species. The specimen here represented, I owe to the kindness of 

 Mr Cooper. It is with pleasure I dedicate this species to my friend, 

 Isaac Hays, M.D., whose talents have been actively and successfully 

 engaged many years in the promotion of natural as well as medical 

 science. 



Unio Hildrethianus. Plate III. fig 



&• 



Testa angusto-ellipticu, subcylindraced, valde transversa, insequildterali; valvu- 

 lis lenuibus; dente cardinali in valvuld utruque iinico, laterali nullo ; margaritu 

 superne fused, in/erne alba et iridescente. 



Shell narrow-elliptical, subcylindrical, very transverse, inequilateral; valves thin; 

 cardinal teeth single in each valve; without lateral teeth ; nacre, above brown, below 

 white and iridescent. 



Hab. Ohio, near Marietta. Dr Hildreth. 



My Cabinet. 

 Cabinet of Dr Hildreth. 

 Cabinet of the Academy of Natural Sciences. 

 Diam -5, Length -7, Breadth 1-6 inches. 



Shell narrow-elliptical, subcylindrical, very transverse, inequilateral, 

 somewhat compressed at basal margin ; substance of the shell thin be- 

 hind, thicker before ; beaks slightly elevated ; ligament long and thin ; 

 epidermis dark brown ; cardinal teeth lobed, single in each valve, larger 

 and wider in the left valve ; lateral teeth none ; anterior and posterior 

 cicatrices both confluent; dorsal cicatrices in the centre of the cavity 

 of the beaks; cavity of the beaks shallow and tinged with dull purple; 

 nacre white and iridescent. 



