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THE AMERICAN NATURALIST [Vol. XLV 



ically and anatomically, an Ancylus, I call the individ- 

 uals "ancyloids." 



In the second stage when the base of the conical shell 

 is more or less closed by a flat horizontal septum con- 

 tinuous with the margin around it, I call the individuals 

 "septates. " 



Lastly, when the animal in its second season begins to 

 form a marginal expansion external to the septum, and 

 with its longitudinal axis sometimes at a considerable 

 angle with the axis of the ancyloid shell, I reserve for 

 this stage, up to and including maturity, the term 

 " Gundlachia." 



Mr. Allen kindly sent alcoholic specimens of ancyloids, 

 septates and Gundlachias from his aquarium for ana- 

 tomical examination. The posterior part of the foot en- 

 tirely hides the septum when the living animal on the 

 walls of the aquarium is examined through the glass. 

 Nothing to distinguish it from ordinary Ana/his is 

 visible in the soft parts. The creatures feed on the 

 microscopic algae, etc., which grow on the walls of their 

 domicile and when feeding the movement of the jaws and 

 radula can be seen with ease by means of a magnifier. 

 On the alcoholic specimens, on the exterior of the shell, 

 were many minute lenticular capsules which, from anal- 

 ogy with Ncritiua, Pom pholt/.r, etc., were supposed to be 

 the ovicapsules. The very young shells are very trans- 

 parent and fragile. It is difficult to find them until they 

 have reached a length of over a millimeter, and so far it 

 has proved impracticable to detach them from their roost 

 without crushing them, they are so extremely fragile. 

 The smallest septate seen was slightly less than two 

 millimeters in length and the animal had entirely with- 

 drawn behind the septum, which .overs more than two 

 thirds of the aperture. 



The species in the Gundlachia stage agrees substan- 

 tially with the form described from the District of Co- 

 lumbia by Stimpson, under the name of Gundlachia 

 meelciana. As in many other fresh-water shells the newly 



