90 



men, and other characters, render it sufficiently 

 distinct. It has, perhaps, more general resemblance 

 to Unio trapezoides than to any other species. 



UNIO HETERODON. 



Plate XLIX.— Fig. 3. 



DESCRIPTION. 



Shell rhomboidal-ovate, ventricose, valves thin; 

 ligament margin short, elevated, parallel with the 

 basal margin; umbonial slope rounded; beaks decor- 

 ticated; basal margin straight in the middle; posterior 

 extremity rounded or very obtusely angulated, a little 

 above the line of the base; within bluish; cardinal 

 tooth in the left valve trilobed; in the right valve, 

 single, elongated, oblique, compressed; lateral teeth 

 slightly curved, double in the right valve. 



SYNONYME. 



U. heterodon, Lea. Trans. Amer. Phil. Soc, new series, vol. 



iii. p. 428, pi. viii. fig. 11. 

 Cab. A. N. S. t No. 20425. 



OBSERVATIONS. 



Inhabits the Schuylkill and other rivers in Penn- 

 sylvania. This interesting little species was first 

 found by Messrs. Mason and Hyde, and my father 

 suggested the name of heterodon, which Mr. Lea has 

 adopted. It is related to U. viridis and compressus, 

 but the double cardinal teeth being in the right valve 

 will distinguish it from every other species. 



