AND OTHER FAMILIES. 139 



also differ so completely as those of the rivers on the two sides of the 

 great ridge ? In these the distinction does not exist, for we find almost 

 every species which is common on the eastern, equally common on 

 the western side. There are, however, some species which are not 

 uncommon on the western side, but which do not exist, so far as my 

 information extends, on this side. If it be demanded why the line of 

 demarcation should not be as perfect for terrestrial as fluviatile shells, 

 we might say in answer, that the barrier of a mountain could in time 

 be overcome even by the slowly travelling snail. Surely in the lapse 

 of time the progeny of those which accidentally began to climb the 

 steeps, might descend into the valleys of the opposite side. 



In finishing these introductory remarks, I wish to call the attention 

 of those naturalists who are conveniently located, to make further ob- 

 servations on this branch of the science, which certainly has great in- 

 terest. 



In describing the Valvata arenifera in my last memoir, ante page 

 114,1 was impressed with the idea, from the circumstance of find- 

 ing a true operculum combined with a spiral tube, that the animal 

 must have belonged to the family Peristomiana. I have reason, how- 

 ever, since, to doubt the truth of my conclusions. Professor Troost, 

 now at Nashville, Tennessee, originally sent the specimens from that 

 neighbourhood ; and from his description of the animal, which he has 

 recently communicated to me, I am induced to believe it to be a spe- 

 cies of Linnean Phrygania. 



