59 



by the probable habits of the animal. All those hitherto discovered 

 of that group, arc known to swim at the surface of the ocean, and 

 not being furnished with other organs of locomotion than fins, they 

 cannot glide upon the bottom ; we must, therefore, (analogically) 

 suppose this to have been the habit of the animal ; and yet it is 

 hardly admissible that it should, in that case, have eluded the 

 observation of voyagers, since the shell has not unfrequently been 

 found in a state of occupancy by the parasite. 



[No. 3, Journal of the Academy of Natural Sciences of Philadelphia.] 



[J. A. N, S. vol. i. p. 13, et seqq. May, 1817.] 



Grenus Cyclostoma. — A subdiscoidal or conic univalve. The 

 aperture orbicular, with a circularly continued margin, often sud- 

 denly and widely reflected. 



Cyclostoma tricarinata. — Shell with three volutions ; three 

 revolving, carinate, prominent lines, giving to the whorls a quad- 

 rate, instead of a cylindric appearance. Suture canaliculate, in 

 consequence of the whorls revolving below the second carina and 

 leaving an interval. Spire convex, apex obtuse. Umbilicus large. 

 Carina placed, one on the upper edge of the whorl, one on the 

 lower edge, and the third on the base beneath. Breadth one-fifth 

 of an inch. 



Inhabits the river Delaware. Rare. Found by Mr. Le Sueur, 

 whose proposed name is here adopted. 



Cylostoma lapidaria. — Shell turreted, subumbilicate, with 

 six volutions, which are obsoletely w^rinkled across. Suture im- 

 pressed. Aperture longitudinally ovate-orbicular, operculated, 

 rather more than one-third of the length of the shell. 



Length about one-fifth of an inch. Collection of the Academy 

 of Natural Sciences. • 



Inhabitant not so long as the shell, pale ; head elongated into a 

 rostrimi as long as the tentacula, and emarginate at tip ; tentacula 

 two, filiform, acuminated at tip, short ; eyes prominent, situated at 

 the external or posterior base of the tentacula; base or foot of the 

 animal dilated, oval, obtuse before and behind. 



Found under stones, &e. in moist situations, on the margins of 

 rivers. Like those of the genera Lymnxa and Planorbis this 



