59 
by the probable habits of the animal. All those hitherto discovered 
of that group, are 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. 
Cyolostoma 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. Bare. Found by Mr. Le Sueur, 
whose proposed name is here adopted. 
Cylostoma lapilaria. — Shell turreted, subumbilicate, with • 
six volutions, which are obsoletely wrinkled 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 
rostrum 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, &c. in moist situations, on the margins of 
rivers. Like those of the genera Lymnsea and Planorhis this 
