198 
The genus Pileohis, Sowerby, has a very prominent labium or 
diaphragm, but the spire is altogether concealed. 
Crepidtjla plana. — Crepidula plana, nob. Journ. Acad. 
Na,t. Sci., vol. 2, _p. 226. 
Ohs. This species is remarkable for its depression, being with- 
out convexity above, and is even often curved upward. In order 
to give space beneath for the substance of the animal, the margin 
of the shell is vertically deflected. The general curvature is various 
in different individuals, being modified in compliance with that of 
the surface on which they rest. PI. 44. 
Modiola. — Shell equivalve, inequilateral, obliquely elongated, 
more or less subovate, regular; summits near the posterior ex- 
tremity, but not terminal ; anterior margin obtusely rounded ; pos- 
terior margin short, rounded ; posterior base a little gaping to 
permit the passage of the byssus ; ligament marginal, rectilinear, 
subinterior, elongated ; hinge edentulous ; two principal, remote 
muscular impressions, of which the anterior one is larger, and secu- 
riform ; palleal impressions entire. 
Ohs. A genus of marine bivalves, separated by Lamarck from 
the Linnman genus Mytillus, in consequence of the beaks not be- 
ing terminal, as they are in that group. It must be confessed, 
however, that they are very closely related, corresponding in the 
other characters of the shell, as well as in the habits of the animal, 
the organization of which, according to the anatomist Poli is simi- 
lar, and to which collectively he appropriates the name of Calli- 
triclie ; for a description of it the reader is referred to the genus 
Mytillus. Several species blended by Lamarck in Modiola, were 
separated by Cuvier under the name of Lithodomus, in conse- 
quence of their habit of penetrating and dwelling in the interior of 
calcareous rocks and other solid substances ; their shell, excepting 
its more cylindrical form, has much the character of Modiola ; but 
it is probable, judging by their mode of life, that the characters of 
the animal will justify the arrangement of Cuvier. 
The byssus is a fascicle of filiform fibres, each of which is sud- 
denly enlarged at tip so as to form a little disk, that is applied by 
the foot of the animal to the supporting object. 
Lamarck described twenty-three recent and several fossil 
species. 
