198 



The genus Pileohis, Sowerby, has a very prominent labinm or 

 diaphragm, but the spire is altogether concealed. 



Crepidula plana. — Crepidula plana, nob. Journ. Acad. 

 JYat. Sci.,vol 2, p. 226. 



Obs. 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. 



Obs. A genus of marine bivalves, separated by Lamarck from 

 the Linnasan 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, con-esponding in the 

 other characters of the shell, as well as in the habits of the animal, 

 the organization of which, according to the anatomist Poll 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 Cuvicr 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 3Iodiola ; 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. 



