330 Variation Exhihited hy a Carbonic Gasteropod. 
tiary Sedimentary formations of the Simla Region of the Lower 
Himalayas. Rec. Geol. Surv. India, xxi. pt.o. Aug. pp. Io0-143. 
— Dunn, E. J. Notes on the Occurrence of Glaciated Pebbles and 
Boulders in the so-called mesozoic Conglomerate of Victoria. 
Trans. andProc. R. Soc. Victoria, xxiv. pp. 44-46. 
— Derby, Orville, A. Mittheilung eines Brief von Herrn A. Derby 
uber Spiiren einer Carbonen Eiszeit in Siidamerika. [Communi- 
cated by Waagen]. N. Jahrb. f. Min. etc. 1888. vol ii. Hft. 2. 
pp. 172-176. 
U. S. Geological Survey, March, 1889. 
VARIATION EXHIBITED BY A CARBONIC GASTEROPOD. 
By Charles R. Keyes. 
It has recently ^ been intimated that among certain species 
of Platyceras there often exists considerable diversity in the 
form of the shell and in the configuration of its aperatural 
margin. Both of these variant features have been attributed 
in great part to certain habits peculiar to the mollusca of this 
and allied groups. Specifically it is the attachment, during a 
greater part of life, of these gasteropods to foreign bodies, and 
especially to the ventral surfaces of paleozoic crinoids. In 
summarizing^ the observations elsewhere more fully discussed 
it was shown that invariably the lip of the calyptrsean shell 
adapted itself exactly to all the inequalities of the surface 
with which it came in contact, and that not unfrequently the 
entire form of the gasteropod shell was more or less deter- 
mined by its'accidental station. In a group so closely allied 
to the modern Capulus and having a habitat like many Pla- 
tycerata manifestly possessed, these phenomena might natur- 
ally be expected ; but the direct evidence as to the extent of 
variability in a single species, and the actual variant range of 
the different characters has never been more than merely sug- 
gestive. Definite results from comparative studies of this 
kind have always been attended by more or less difficulty 
arising from divers sources. When extreme forms are pre- 
sented there is always a slight suspicion that in reality they 
may not be specifically related ; and when the comparison is 
extended to specimens from localities widely separated geo- 
graphically, and perhaps also geologically, the problem of 
specific identity becomes even more highly complicated. A 
large series of Platyceras eqitilaterum Hall from the blue 
^ Keyes. Proc. Am. Philosophical Soc, vol. xxv, p. 240, 1888. 
'Keyes. Am. Jour. Sci,, vof. xxxvi, pp. 269-272. Also, Am. Nat- 
uralist, vol. XXII, pp. 924, 925. 
