121 



" I have had my attention called to a peculiar form of the 

 Alasmodon marginata^ Say, from Ohio, (known as A, truncaLa^ 

 Say,) by a remark in a letter from one of my correspondents, 

 who says, that ' Mr. Lea is of the opinion, that this truncata is the 

 female.^ The same shells are found here in the Mohawk River, 

 and are all about one form, without any appearance of the trun- 

 cation or shortening of the posterior, often seen in shells from 

 Ohio. Having examined the animals as well as the shells, I am 

 prepared to say, that the female differs from the male in form, 

 only in being a very little more inflated. A careful examination 

 of some Ohio shells, very much truncated, leads me to the con- 

 clusion that the shortening is incident to the shell only in certain 

 localities, where the currents bear along substances that, coming 

 in contact with the edges of the valves of the shell, break them 

 down ; and the result is, the growth of the shell is retarded at that 

 point, and the efforts of the animal are directed to the renewal of 

 the broken portion, while the growth of the shell proceeds regu- 

 larly in those portions which are concealed in the mud, in which 

 the animal partially buries itself. An inspection and comparison 

 of the two forms of the shell will show the correctness of the 

 above views. 



" I have been anxious for some time to ascertain whether those 

 shells known as Lymnea reflexa, and L. ujnhrosa, are confined 

 to any particular geological sections, and, if so, what is the na- 

 ture of those sections, or regions ? I have some suspicion, that 

 they are not specifically diflierent from those shells we call Z. 

 elodes ; but as I have no localities furnishing these shells, I am 

 not able to arrive at any conclusions. 



Physa. 



" In comparing shells known as P. elongata. Say, from Massa- 

 chusetts, New York, Ohio, and Michigan, I think I discover cer- 

 tain differences which may have escaped observation heretofore. 

 The shells from Michigan have a suture somewhat flattened, 

 like the suture of P. ancillaria, Say, while those from Massa- 

 chusetts and New York have a very deeply impressed suture, 

 ahd are smaller, &c., d:c. 



" If a further examination and comparison of these shells 



