34 AMERICAN JOURNAL 



when perfect, acute. Opercle gibbous ovate. Examples : 31. 

 decisa, Say, M. Milesii, Lea, M. fecunda, Lewis, M. lima, 

 Anth. 



Fourth. A group embracing only M. De Campii, Binney. 



My examination of Mr. Wheatley's collection destroys the sym- 

 metry of this arrangement in some degree, and suggests some 

 new views respecting a few of the species named. Among Mr. 

 Wheatley's shells were a number from the Coosa River, Ala- 

 bama, which had been labeled by the correspondent from whom 

 he received them M. coarctata, Lea. A hurried glance at these 

 shells would suggest only to the observer their apparent identity 

 with the more mature forms usually regarded as poyuhrosa. 

 Two of these specimens, however, on careful examination, re- 

 vealed a remarkable resemblance to the figure given by Mr. 

 Binney in connection with his quotation of Mr. Lea's description 

 of coarctata. They agree also with the description in all essen- 

 tials, and, so far as is known, are the only shells found in " Ala- 

 bama "to which Mr. Lea's description and the accompanying 

 figures given by Binney will apply. Various shells found in dif- 

 ferent portions of the valley of the Mississippi heretofore doubt- 

 fully referred to coarctata and to exilis (Anth.) differ enough 

 from the figure. 



We will add to this that Mr. Lea, in citing locality, gives only 

 vaguely " Alabama." The specimen, a single one, was in the 

 collection of Dr. Foreman, whose name at various times appears 

 in connection with species "from the Coosa River," described 

 by Mr. Lea, in such manner as to give countenance to the sujjjjo- 

 sition that Melantho [Pal.) coarctata. Lea, may have been derived 

 from that prolific stream. All the probabilities of the case 

 point very strongly to the supposition that the true coarctata 

 is a young shell from the Coosa River, which, when mature, re- 

 ceives the name ponderosa. 



All the shells from the Coosa River that are regarded as un- 

 questionable ponderosa by collectors, have a peculiar appearance 

 by which persons familiar with them may separate them from 

 similar shells of the rivers of the Ohio system. The young shells 

 of the ponderosa of the Ohio system do not, so far as I have 

 any knowledge of them, ape those forms that may be identified 

 with coarctata. It is a generally received principle in Natural 

 History that marked difi'erfoces in the embryos and young of a 

 class of beings are specific. If we apply this rule to ^''pon- 

 derosa " of the Coosa (referring to the peculiar forms assumed 

 by immature specimens,) it will be a proper inference that those 

 shells are specifically distinct from the ponderosa of the Ohio 

 system. 



