37 



founded under the general name o^ purpureas. Mr. 

 Mason, who has more thoroughly investigated these 

 shells than any other Conchologist, has brought 

 together in his cabinet specimens so dissimilar in 

 appearance as to perplex the student if he admits 

 them as varieties only of one species; for in that case 

 he must extend his scepticism to a number of other 

 shells, hitherto admitted as distinct species. Differ- 

 ent localities, no doubt, produce some corresponding 

 difference in the general appearance of certain shells, 

 a remarkable instance of which is exhibited in U. 

 niger, Raf., which in Flint river is quite a dwarfish 

 shell, compared with the same species in other 

 rivers. This remark will apply equally to U. costa- 

 tus, Raf. of Flint river; but the specific character is 

 not lost in the varieties of these two shells, which 

 cannot be said of the so called varieties of U. purint- 

 reus. Perhaps U. coarctatus, Lam. will prove a dis- 

 tinct species, but I leave it for a more accurate ob- 

 server to make further comments on this difficult 

 subject. No one could do it more justice than Mr. 

 Mason. 



UN 10 PECTOROSUS. 

 Plate Y\.—Fiir. 1. 



DESCRIPTION. 



Shell oval, ventricose, not thick; posterior margin 

 broad, biangulated; umbonial slope subangulated; 

 space behind flattened, with an impressed line passing 

 from the beak to near the middle of the posterior 

 margin; anterior side slightly produced; umbones 



D 



