AND OTHER FAMILIES. 137 



the Ohio, certainly has a different aspect from that of our eastern rivers, 

 and might with great propriety be referred to the name which Mr Say 

 gave it long since, viz. U. crassus.* There is another shell, however, 

 in the Ohio, which has a stronger resemblance than this, and I believe 

 it to be the analogue of the Alasmodonta morginata (Say). I have 

 examined numerous specimens of this species frequently and attentively, 

 but cannot distinguish any difference except in the size, the western 

 shell being generally much larger. Of the numerous species and 

 genera of the families Lymneana, Melaniana and Peristomiana, I have 

 never seen a single species common to both waters. To the genus 

 Cyclas I have given but little attention, but believe the same observa- 

 tions may be extended to this species. 



What an interesting field do these facts spread open to the inquiring 

 philosopher! Why should the streams which flow down the sides of 

 the same range of mountains, east and west, differ so essentially in 

 their productions ? 



Let us now examine the extremities of this great chain. To the 

 north, where it is lost in the high lands which spread out along the 

 southern boundaries of the small lakes in the state of New York, great 

 difficulty naturally occurs in defining the line of separation. So far as 

 my observation has extended, the shells of the river Mohawk and its 

 tributaries are the same with those of the Delaware, Potomac, &c. with 

 the exception of a single species Symphynota compressa (nobis), which 

 is found near Albany, and which exists also in the Ohio. The tribu- 

 taries of the lakes Erie, Michigan, &c., with few exceptions, produce 

 the western species, and consequently the lakes do also.f 



The great river Niagara, or rather strait connecting the lakes Erie 

 and Ontario, furnishes us with the U. triangularis (Barnes), and other 

 species, which are so peculiarly characteristic of our western waters. 

 Never having visited the shores of lake Ontario, I cannot pronounce on 

 its productions. The shells of the river St Lawrence are, I believe, 



* A shell which I have always considered as a truncated variety of U. crassus of the Ohio, 

 has by this naturalist been made a new species under the name of U. abruptus. 



t Since writing the above I have received the V. complanatus (Soland.) from Lac Vaseux, 

 which empties into Green Day; and more recently the same species from lake Champlain. 

 The U. nasutiis (Say) has been observed in Grand river, which disembogues into lake Erie. 

 37 



