36 



boundary of the United States, beyond which, I believe^ 

 none have been found. The Oliio river seems to form- 

 a dividing line also, both northern and southern species 

 fading out as they approach this stream. Tliey are 

 not found in New England at all, nor in the vicinity of 

 the Ocean. The distribution of specnes and the char- 

 acters of the shells of the north find a parallel in some 

 marine mollusks, and leave no doubt as to species 

 being specially adapted to the station they occupy in 

 the first creation, though some of them may be sub- 

 sequently modified by change of the conditions. The- 

 family were formerly called Melanlans from their sup- 

 posed affinity to Orientiil forms ; and it was not until 

 recently that the differences both in the shell and ani- 

 mal were pointed out by Prof. Haldeman. Our species 

 have a plain or entire margin to their mantle, whilst 

 the Oriental species have a fringed mantle. The soft 

 parts have not, as yet, been much studied in this coun- 

 try, and species, genera, etc., are based upon the char- 

 acters of the shell. We have in this county four 

 species, three of them very abundant in all our rivers, 



GENUS PLEUROCERA, RAFINESQUE. 



PI.EUUOCERA SU1IIJI.A.RK, Lea. (Trypanostoma.) 



The following description is from American Journal 

 of Conchology, Vol. 1: " Acutely turrited ; rather 

 thin ; spire much elevated ; apex acute ; whirls twelve, 

 fiat ; carinate at apex ; body whole ; angulate on the 

 periphery. Horn color, generally light yellow and 

 bluish ash below the suture." 



