Keyes, ] 240 (Oct. 19, 
to be no serious objections to this inference ; but with the univalves their 
position through life indicates that their food was in great part at least of 
an entirely different nature, and this is one of the considerations that 
suggest a possibly much wider generic separation of Platyceras and Cap- 
ulus than is apparent from a comparison of the shells. The anatomy of 
the crinoid and the position of the molluscan shell are opposed to the sup- 
position that the gasteropod may in any way have been nourished on the 
food of the crinoid. This would imply that the mollusk was parasitic in 
its habits, a view which, though held by some writers, does not appear to 
be structurally sustained. While no doubt the Platyceras derived the 
greater portion of its food from excrementitious matter, nourishment from 
other sources may also have been obtained, and in all probability it was 
very similar to that of the crinoids and living Calyptreide. Furthermore 
there does not seem to be the slightest indication that the crinoid was in 
any manner inconvenienced by the attachment of the gasteropod, except 
perhaps in a few cases where the molluscan shell has encircled the postero- 
Jateral arms which were in consequence slightly pressed outward. The 
only really noticeable effect of the presence of Platyceras on the crinoid 
is a comparatively shallow depression or groove on some of the vault 
plates—marking the position of the shell margin; yet in the majority of 
specimens this feature is scarcely perceptible. 
In considering the structural peculiarities of the Platyceras shell three 
features—the general form, the configuration of the aperture, and the sur- 
face markings—appear to have been susceptible of considerable modifica- 
tion as the result of the sedentary habits of the mollusk. An examination of 
a large series of certain species of Platyceras reveals that the variant ten- 
dency in all three of these particulars is much greater than might be sup- 
posed, but when the attachment of these gasteropods to foreign bodies is 
taken into consideration the causes for such varietal development become 
manifest. It has been shown that the mollusk doubtless remained fixed 
throughout a greater portion of life, and that the surface upon which it 
first settled determined in great part both the form of the shell and the 
shape of its aperture. When the surface of attachment was flat, as in the 
vaults of Ollacrinus and Strotocrinus, the shell was greatly depressed and 
the peristome ample, but when the foreign body was strongly convex the 
shell was more conical with a comparatively much smaller aperture. It 
has been shown elsewhere that, in regard to the second of the three variant 
features observable in the calyptreean shell, the margin of the peristome 
partakes of all the inequalities of the surface to which the gasteropod 
adheres. Few of the species attached to crinoids may be said to have true 
surface ornamentatiom, but the longitudinal folds or plications in tbe shell 
are in many cases due largely to the character of the surface of attach- 
ment. In some specimens of Platyceras infundibulum M. and W. there 
has been observed in addition to the undefined longitudinal folds, series of 
small conspicuous nodes ; but in all examples these nodes appear to have 
resulted from the peculiar nodose ornamentation of Platycrinus hemi- 
sphericus with which these univalves were associated. 
Ye 
