LIMNEA DESIDIOSA. 33 



diameter and two feet deep, on the Susquehan- 

 na, near my residence. It is subject to desic- 

 cation in very dry seasons, and has a bottom 

 of mud, and but little vegetation, which is 

 chiefly confervoid. The soil is slightly cal- 

 careous. I am thus particular, because this 

 pond appears very favourably adapted to the 

 growth of these animals, as well as Physa 

 heterostropha and Planorbis bicarinatus. I 

 have never found but the single specimens 

 with as short a spire, or as wide an aperture, 

 respectively, as those represented by 5 and 7. 



Having received specimens of his 'L. acuta,' 

 fig. 14, and 'Philadelphica,' fig. 10, from Mr. 

 Lea, I have no hesitation in referring them to 

 this species, to which his L. casta probably 

 belongs also. 



Explanation of Plate 10. 



The shells figured are all Pennsylvanian. Figures 2 

 and 3 represent specimens from Tuscarora creek, and may 

 be assumed as good examples of the species; 10 and 12 

 from near Philadelphia; 11, 13, 15, from near my resi- 

 dence; and 12, from the gutters along the road above 

 Columbia. The last bears a very close resemblance to 

 Say's obrussa, but I cannot, with certainty, refer the latter 

 to the species under consideration, because I have not a 

 sufficient number of authentic specimens for comparison. 



K 5 



