187 



more or less of the exterior surface of the shell, and is digitated on 

 its margin, and the tentacula, which are short and compressed in 

 Limneus, are longer and almost filiform in the Phi/sse. The shell 

 also of the latter may be distinguished by 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 Physa. Mixas of Leach may also, perhaps, be separated from 

 Limneus ; it was formed for the L. ghitinosus, 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 subject 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 umbrosus. — Limneus elongatus, noh. Journ. Acad. 

 Nat. Scl, vol. 2, p. 167. 



■Ohs. It inhabits in considerable numbers the ponds and tran- 

 quil waters of the Missouri, in the vicinity of Council Blufij and 

 Bigsby obtained specimens in Rainy lake and Seine river of Upper 

 Canada. 



