xlix 



A peculiar group of shells, j^ossessing an inflated 

 form and much lighter texture, is found in the 

 Potomac and Susquehanna Rivers, the Kanawha 

 and the upper Ohio. They are — A. dissimilis, 

 dilatata, costata and tril'meata. 



Concluding Ohservaiions. 



In studying the species of Strejmmatidce, especial 

 care must be taken not to consider young shells to 

 be adult species. All of our conchologists who 

 have described species of this family have fallen into 

 this error. The aspects assumed by young or 

 half-grown shells are frequently so very difterent 

 from their appearance when mature, as to be liable 

 to mislead experienced naturalists. 



All quite young shells are characterized hy a thin 

 texture, very light color, and very sharp acuminated 

 spire, and in most cases by the base of the aperture 

 being acuminate also. 



J^early every species^ even when smooth in its 

 adult state, presents the first few wdiorls either 

 sharply carinate, or plicate, or striate. Occasionally 

 they are either one or the other in the same species. 

 Hence, in describing shells as carinate, or plicate or 

 angulate, the appearance presented by the adult 

 only should be thus described. 



In some of the species, however, these lines, 

 plicae or carinae, are persistent in the old shell, under 

 favorable circumstances, but in most specimens are 

 not seen. This is one difficulty which has caused 

 the multiplication of synonymic names, generally 

 unavoidably, on account of the scarcity of spec- 

 imens, known to be from the same locality, for 

 comparison. 



When a specimen exhibits a perfect spire in the 

 adult state (rare among the Sfrej^oinatidce) and the 

 initial whorls are plicate or carinate, they cannot be 

 regarded as affording reliable data for specific dis- 

 crimination. And it is only when these marks 



L. P. W. S. IV. 



