Fresh-Water Shells. 209 



Planoebis Dilatatus. 



(^ I ^ ^1 t^liell small, of a yellow isli green-color, 



\m ^ ^ W^ ininutelywrinkled by the lines of growth; 



spire flat, composed of not more than three whorls, separated by 

 a well-defined suture ; the outer whorl has a sharp margin on a 

 level with the spire, diminishing near, but still modifying, the 

 aperture ; below this line the Avhoi-] is very convexly rounded so 

 as to encircle a small, deep, abruptly formed umbilicus. This 

 whorl rapidly enlarges, and terminates in a very large, not very 

 oblique aperture, with the lip expanded so as to make it trumpet- 

 shaped. Largest diameter ^\ inch, breadtli ^V i"P^- 



It has a miniature resemblance to P. bicarinatus as to its two 

 sides, but it has only a single carina, which encircles the shell, 

 instead of one on each side. Its large, expanded aperture, and 

 small, deeply sunken umbilicus, readily distinguish it from any of 

 the small species hitherto known. The surface is ratlier rough, 

 and perhaps a little hispid when viewed under the microscope. 



The P. lens of Lea (Amer. Philos. Trans., JVew Series, vi, 68, 

 pi. 23, f. 83,) which he received from near Cincinnati, is probably 

 the same as this shell. His name, however, is pre-occupied by a 

 fossil species. 



Genus PHYSA, Drap. 



Shell reversed, oblong-ovate, spire jiromineiH ; aperture rounded 

 before,narroioed and angular behind. Up sharp ; inner lip tioisted 

 Animal has thread-lilce tentacula, and the sharply lobed mantle, 

 is turned back upon the shell. 



Physa Heterostropha. — (Say.) 



" Shell sinistral, sub-ovate ; color pale-yel- 

 low, chestnut, or blackish; whorls four, the 

 first large, the others very small, tei-minating 

 rather abruptly in an acute apex ; aperture 

 large, somewhat oval, three fourths the length 

 of the shell, or rather more; within of a pearly lustre, often 

 blackish ; lip a Httle thickened ou the inside, and tinged with 

 dull red." (Say, in Nich. Encye.) Ordinary length about \ 

 inch, breadth \ inch, divergence CS"^. My largest specimen is y\ 

 inch by f inch. 



When the shell is fresh and perfectly clean, it is always of a 

 light greenish-yellow, and becomes a little more dusky with age. 







