252 



This septum would In some degree serve as a protection to the 

 Mollusk during this period, in the same way as the epephragm of the 

 Helices. 



In the following spring, — the period of greatest activity in growth 

 with all the fresh-water Pulmonates, — the animal throws forth its new- 

 er and larger shell, retaining the older one on its back for the protec- 

 tion of its more tender viscera. It therefore wiU be a matter of great 

 interest and importance to observe these animals in the latter part of 

 winter, when the formation of the newer shell Is about to commence. 

 At that period, they will be found to present the primary form, namely, 

 that of an Ancylus with two-thirds of its aperture closed by a septum, 

 leaving but a small opening for the egress of the foot of the animal. 



This remarkable little MoUusk, of a genus new to our Fauna, has 

 occurred to me in one locality only, — a small pond of clear water, In a 

 marshy bank of the Potomac, on the northern side, between George- 

 town and the Little Falls in one direction, and between the canal and 

 the river in the other. The pond is about one mile below the so-called 

 " Chain Bridge." Five specimens only were found, after repeated 

 search. 



I have dedicated this species to my friend, Mr. F. B. Meek, the most 

 accurate of American Investigators In Fossil Conchology, the pleasure 

 of whose company I enjoyed during several excursions for the purpose 

 of procuring specimens of It. 



The Secretary read a communication by Dr. James Lewis, 

 of Mohawk, N. Y., on the identity of Paludina subglobosa 

 Say with Melania isogona Say. 



Dr. Wyman stated that he had recently observed the 



gest a careful repetition of the observation. For if the young animal when hatched 

 has a septiferous shell, of the same form as at the period when the newer shell is 

 about to be thrown off, it is difficult to conceive how its enlargement by growth in the 

 first season is effected. Shells increase in size by additions to the margin of their ap- 

 erture, but such additions to the lips of the little semi-circular aperture of the young 

 Gundlachia would simply close it up entirely. The alternative — that the shell when 

 hatched has already reached the size (one-eighth of an inch in length) seen in the 

 immature septiferous individuals, and that they increase no more until the larger 

 shell is thrown off — would seem to be contrary to all that we know of the earlier 

 condition and growth of other genera of the family, such as Ancylus. I should 

 mention, however, as an argument in favor of Bourguignat's view, that I have 

 been unable to detect distinct lines of growth upon the superior portion of the 

 shell ; but, on the other hand, there are very distinct lines of growth upon the sep- 

 tum^ which would indicate that that portion of the shell at least was formed after 

 the birth of the animal, for such lines of increase are not seen in shells formed in 

 the egg. The specimens first found in Cuba, which, as before-stated, were imma- 

 ture and septiferous, occurred in a dried-up pond, the dry season in that climate 

 corresponding in its effects upon Mollusks, as well as in time, to our winter. 



