128 
[No. 4. Long’s Expedition, &c. vol. ii. p. 261, et seqq. 1824.] 
Planorbis defleotus. — Shell dextral, depressed; whorls 
nearly five, minutely and regularly wrinkled across, wider than 
long, with a much depressed rotundity above, descending to an. 
acute lateral edge below the middle ; spire not impressed ; suture 
indented, but not profoundly; beneath a little concave in the 
middle, exhibiting one-half of each volution to the apex ; whorls 
flattened, slightly rounded ; aperture declining very much, sub- 
oval, the superior portion of the labrum considerably surpas- 
sing the inferior portion, and taking its origin a little above the 
Carina; inferior portion of the labrum terminating on the middle 
of the inferior surface of the penultimate whorl. 
Greatest breadth two-fifths of an inch. 
This shell was presented to me by Pr. Bigsby, who collected 
many specimens in the waters of the North-west Territory. 
It resembles the exacuous^ nob., but the aperture does not em- 
brace so large a proportion of the preceding volution, and the 
volutions on the inferior portions of the shell are consequently 
more obvious, and the umbilicus is but slightly indented ; the 
upper portion of the labrum does not extend so far beyond the 
lower portion, the aperture declines much more, and the carina is 
less acute. It has also an affinity for the carinatus of Europe, 
but in addition to other differences, the aperture of that species 
declines but little, if at all, and the carina is an elevated revolv- 
ing line. The aperture embraces the penultimate volution about 
as much as in the rotundatus of Europe, to'which our shell is also 
allied, but differs in its declining aperture, and the less degree of 
rotundity of its whorls on their upper surface. PI. 15, fig. 8.* 
Planorbis corpulentus. — Shell dextral ; whorls more than 
three, rather rugged with coarse wrinkles, much higher than wide ; 
superior surface much flattened, and edged by an abrupt acute 
line, which is distinct to the aperture ; sides hardly rounded, and 
terminating below by another abrupt edge, which is not quite so 
definite and acute as the superior one; spire slightly concave; 
umbilicus exhibiting a portion of each of the rapidly retiring 
whorls to the apex ; aperture longer than wide, the superior part 
extending higher than the preceding volution, and the inferior 
* [Plate 74 of this reprint. — E d.] 
