GLNDLACHIA. 151 



dentition. This resembles very nearly that of a species of Ancylus com- 

 mon in the District (which appears to be the A. rivularis of Say and Hal- 



Lingual dentition of Gundlacliia meekiana. 



deman), differing from it only in having two or three teeth less in number, 

 and in the more numerous denticles with which its lateral teeth are 

 armed. 



After a close examination of the above characters, I have ventured to 

 suggest that the Gundlachia commences its life as an Ancylus; the smaller 

 shell, in which the earlier period of its life is spent, being undislinguish- 

 able in form from the shells of that genus. It is probable that it passes 

 the first summer and autumn of its existence in this smaller shell and 

 that the septum which afterwards partially closes its aperture is formed 

 during the period of inaction which ensues during the winter. 



This septum would in some degree serve as a protection to the mollusk 

 during this period, in the same way as the epiphragm 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 newer and 

 larger shell, retaining the older one on its back for the protection of its 

 more tender viscera. It therefore will 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 mollusk, 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 Georgetown 

 and the Little Falls in one direction and between the canal and the river 

 on 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. {Stimpson.) 



Gundlachia meekiana, Stiiipson, Proc. Bost. Soc. 1863, 249, fig. 



11 



