354 TRANSACTIONS OF THE WAGNER FREE 



Conrad does not subsequently mention it in his lists of Eocene species, I sus- 

 pect it proved to be the broken tip of some other gastropod, and should be 

 erased as a species. 



Genus CRBPIDULA Lamarck. 



Crepidtda Lam., Prodr., p. 78, 1799. 



Crypta Humphrey, Mus. Calon., p. 4, 1797 ; name only. 



Proscenula Perry, Conch., pi. 53 and expl. text, iSii. 



Humphrey's name in an auction catalogue without figure, diagnosis, or 

 reference to literature, is not entitled to stand against the properly proposed 

 designation of Lamarck. The genus contains three sections, Crepidula s. s., 

 with C. fornicata as the type, lanaciis Morch, with C. ungidfonnis Lamarck 

 as the type, and Sajidalhmi Schumacher for such species as C. aciileata L. 



Beginning in the Claibornian, we have the (". /zV^^z Conrad, a well-marked 

 striated species, of which a very exceptional spinose individual was named by 

 Conrad C. duiiiosa. C. cormiarietis Lea is a synonym of lirata. 



In the Miocene the genus exhibits luxuriance both in size and variation. 

 It is a singular fact that many of the Miocene specimens are bored by carniv- 

 orous gastropods, while I do not remember ever having seen a recent speci- 

 men which had suffered in this way. 



In discussing the characteristics of the forms of Crepidida with the idea 

 of differentiating groups which may have the rank of species, it is necessary to 

 take into account the influence of the environment, which, as Crepidula is a 

 sedentary animal, is particularly strong in this case. It has already been ob- 

 served that the station in perching mollusks is very influential in affecting their 

 characteristics. On a careful inspection of large multitudes of specimens it 

 will be seen, as a rule, that those which have the plane of their pedestal 

 markedly convex and narrow are not only necessarily compressed and dor- 

 sally high-arched, but that they tend to have the apex elevated from the basal 

 margin and the cavity of the beak ventral to the septum prolonged over the 

 posterior margin. In the Gulf of Mexico a form of Crepidula perches on 

 Cerithiuni muscanun. It cannot grow larger, basally, than its perch ; owing to 

 the irregularity of the shell-surface, to which its base exactly conforms, it 

 would almost certainly perish if it tried to seek another siUis. It fulfills its 

 growth by becoming especially high and arched in the back, and the beak both 

 above and below the septum is very deep, though the aperture is small. It is 

 obvious that its opportunities for growth are narrowly limited. Its food must 

 also be limited to that common to the limited haunts of the Ccrithiu7n. As we 

 know in some cases the color of the food directly affects the individual, dynamic- 

 ally and with no delays for natural selection, so in this case such effect on its 

 characters as may be due to food would also be limited. The Cerithium is a 

 rather active animal, and to retain its foothold the Crepidtda must have a good 

 development of its pedal muscle. Since this muscle is so attached that the sep- 

 tum to some extent serves as a fulcrum, it is obvious that a direct effect is 



