18 



This species in some of its varieties so nearly re- 

 sembles U. heros, of Say, when the latter is old and 

 eroded on the umbones, that Mr. Say at one time 

 considered them identical; the young shell of the he- 

 ros, however, was found to be so entirely dissimilar 

 to the young of the costatus, that the specific differ- 

 ence was obvious to the most careless observer, and 

 Mr. Say in his synonymy retains his species. This 

 circumstance should induce those naturalists who 

 live in favourable situations to study the young of 

 every species, for as Mr. Lea has justly observed, 

 they are the most certain guides to specific distinc- 

 tions; when the umbones and beaks are perfect, most 

 species will exhibit very nearly the prominent cha- 

 racters of the very young shells, and hence the ne- 

 cessity of procuring cabinet specimens of the most 

 perfect description, if we would effectually trace the 

 species and make ourselves acquainted with their 

 affinities. 



