54 



I have a pleasure in dedicating it. The outline of 

 a young specimen is very similar to a variety of I). 

 dilatatus, but the thinness of the shell, colour of in- 

 terior, and particularly the form of the teeth, will 

 readily distinguish it. The old shell might perhaps 

 be mistaken for a variety of the U. rectus, but the 

 young has very little resemblance to that of the latter. 

 U. dilatatus is fomid in company with this species, 

 retaining all the peculiarity of habit which it pos- 

 sesses in the Ohio and other tributaries of the Missis- 

 sippi, which confirms our opinion that the shell we 

 described above is new and distinct. 



UNIO LAPILLUS. 



Plate XXIX.— Fig. 2. 



DESCRIPTION. 



Shell suboval, thick, yellowish, with very numerous 

 dark green undulated rays; anterior margin not very 

 short, rounded; beaks but little elevated, simple; liga- 

 ment margin declining gradually in an arcuate line; 

 posterior extremity rounded; umbonial slope unde- 

 fined; basal margin straight or slightly contracted in 

 the middle; within pearly white; cardinal teeth direct, 

 thick; lateral teeth thick, oblique; cicatrices very deep 

 and rounded; cavity of the beaks almost obliterated. 



SYNONYMES. 



U. lapillus, Say. Trans. Journ. of Med. vol. iv. p. 528, Amer. 

 Conch, pi. 41. 



