332 Variation Exhibited by a Carbonic Oasteropod. 
The suggestion as here presented, though affecting but a 
single one of many similar cases in this and other zoological 
groups is of much significance in its bearing upon the real 
basis of species. The vast synonymy existing in nearly all 
departments of paleontology has arisen chiefly from inatten- 
tion to variation as the result of individual environment; and 
the invalidity of a large number of now recognized species is 
clearly evidenced by a comparison of specimens having appar- 
ently the same genetic origin, either from identical or from 
widely separated localities and from different horizons. Else- 
where ^ it has been shown that, in general, shells oi Platyceras 
attached to flat vaults of crinoids have a tendency to become 
very much depressed ; while those adhering to convex surfaces 
are relatively much higher. On the other hand several of the 
specimens examined illustrate another phase of variation 
which has not until quite recently been thoroughly under- 
stood ; a phenomenon which suggests that accidental station 
is not the only factor in modifying the form of the shell, but 
that gravitation also exerts a potent influence. In nearly all 
of the numerous examples noticed in which Platyceras equi- 
laterum is intimately associated with crinoids, the echino- 
derms have been of the type of Strotocrinus and Ollacrinus, 
genera in which the vault is comparatively very large, nearly 
flat, and having the anal opening eccentric in position. In 
several instances, however, P. eqtiilaterum has been found 
attached to the calyx of Platycrinus hemisphericus M. & W. 
from the same blue shales at Crawford sville, Indiana, in which 
occurs Ollacrinus tuberosus Lyon and Casseday. The Pla- 
tycrinus presents some marked structural differences to Olla- 
crinus. The vault is very much elevated, nearly hemispheri- 
cal, with the anal aperture situated laterally between, and 
slightly above, the bases of two arms. The Platyceras usually 
attached to this crinoid is P. infuridibidurn M. & W. a, more 
or less elongate conic species . P. equilateruni when occupy- 
ing the same position is pendant, the apex of the shell being 
directed downward instead of in the opposite direction as 
when resting on the ventral surface of Ollacrinus. The shell 
thus pendant exhibits decided tendency to straighten out, or 
uncoil, consequently becoming longer, and the apex freeing 
itself entirely from the body whorl. P. infundihulum M. & W. 
^ Proc. Am. Philosophical See, vol. xxv., p. 240. 
