476 Recent freshwater Mollusca, 



acquainted with, yet, when we find but a single ex'ception 

 among hundreds of species on both sides, I cannot think it 

 unimportant to inquire whether we are to consider it a mere 

 coincidence, that this anomalous Fusus fluviahs should occur 

 in a stream which rises in a salt region. 



I have noticed, in another place, the great analogy existing 

 between the aquatic Paludina subcarinata and the terrestrial 

 genus Cyclostoma. In the former genus, the rim of the 

 aperture lies in close contact with the body of the shell, and 

 the opercle is composed of concentric elements, whilst, in the 

 latter, it has a tendency to disunion, and the opercle is con- 

 stantly subspiral. In the species in question, " we find a true 

 Paludina occasionally rounding its aperture, throwing it off 

 from the body whirl, and not only assuming the physiognomy 

 of Cyclostoma elegans, but actually departing so far from the 

 normal character of its genus, as to construct a cyclostomoidal 

 opercle, that of the young enlarging spirally. This, however, 

 does not continue ; the animal, as a Paludina, is not endowed 

 with the elements necessary to produce the entire opercle of 

 a Cyclostoma; so that, after continuing it to a certain point, 

 the layers become normal or concentric, apparently indicating 

 the impossibility of a departure from its generic type beyond 

 a certain limit." * When I detected this peculiarity, I consid- 

 ered it as affording an excellent illustration of the theory of 

 " representation ; " and, although I still entertain the same 

 opinion, I cannot close my eyes to the fact that representation 

 itself must take its place as an argument in favor of Lamarck's 

 views. 



Some authors, having adopted certain views of species 

 and varieti s almost at random, and independently of zoologi- 

 cal or physiological considerations, call in the lamarckian prin- 

 ciple as far as their notions render its assistance necessary, 

 and chiefly that they may refer the members of the hu- 

 man race to one zoological species, whilst they will admit that 

 the American Vulpes fulvus is distinct from its European 



* Freshwater Univalve Mollusca, Phvsad^, p. 17. It will be remembered that 

 Lamarck, at one period, confounded the genera Cyclostoma and Paludina, 



