187 
more or less of the exterior surface of the shelly and is digitated on 
its margin, and the tentacula, which are short and compressed in 
Limneiis, are longer and almost filiform in the Pliysse. The shell 
also of the latter may he distinguished hy being heterostrophe. 
For these reasons we agree with Draparnaud and most modern 
conchologists in separating Physa as a genus. It would seem also 
proper to separate Aplexa of Fleming, the animal of which is des- 
titute of a dilated mantle, though the shell itself resembles that 
of Pliym. Mixas of Leach may also, perhaps, be separated from 
Limneus ; it was formed for the L. glutinosus, Drap., which is a 
fragile, vesicular shell, the spire hardly elevated, dextral, and the 
mantle of the animal is reflected. 
These shells are abundant in most of our lakes and ponds, as well 
as in the rivers, east of the Alleghany Mountains. In our Western 
streams they are less frequent, and even in some of them are of 
rather rare occurrence. As the animal can only respire air un- 
mixed with water, it is under the necessity of residing near the 
surface or shore to obtain it. Which, as its motions are slow, it 
cannot do in rivers that are subj ect to very great, sudden and long 
continued changes in depth, as is remarkably the case with our 
Western streams. In order to take in a supply of air the respira- 
tory orifice is opened at the surface with an audible snapping 
sound, like that produced by the resilience of the nib of a pen. 
Like the animals of some other shells it is capable of sustaining 
itself at the surface of the water in a reversed position, the shell 
being downward, and thus it can glide along to a considerable dis- 
tance, by a slow movement. It does not appear to be capable of 
rising from a depth to the surface voluntarily ; but it gradually 
reverses its position from a supporting body that meets the surface, 
or that approaches it within reaching distance. 
Lamarck has described only twelve recent species, but many 
more have been since made known and several fossil species have 
also been described. 
Limneus umbeosus. — Limneus elongatus, nob. Journ. Acad. 
Nat. Sci.j vol. 2,, JO. 167. 
Ohs. It inhabits in considerable numbers the ponds and tran- 
quil waters of the Missouri, in the vicinity of Council Bluff, and 
Bigsby obtained specimens in Bainy lake and Seine river of Upper 
Canada. 
