Casey — Notes on the Pleurotomidae. 127 
necessary, and it may be remarked in passing that the general 
habitus of the shell is frequently a more important criterion 
than any one of them. For example, it may be correctly 
assumed that the characters of the embryo are of great 
weight in an estimation of genera, but I find the variations of 
this part of the shell must be used with very much more cau- 
tion than might be supposed necessary, especially in the 
paucispiral and multispiral conditions, which are of them- 
selves by no means uniformly indicative of generic difference. 
In some genera such as Gemmula Weink. (=Hemipleurotoma 
Coss.) having a rather complex embryo of some five or six 
whorls, the lower one to three of which are finely costulate, 
there is a remarkable persistence and stability of form and 
no perceptible change in type, either in the embryo or general 
characteristics of the shell, from the earliest Eocene to the 
present time, and the wholly extinct Microdrillia Csy., having 
a similar embryo, is also constant during the much shorter 
geological period spanning its history, while in others the nor- 
mally multispiral embryo may frequently become paucispiral, 
sometimes as a result of progressive degradation, but in sey- 
eral instances in consequence of varied environment alone. 
Of this inconstant type of embryo I have in mind three 
striking illustrations. The first is afforded by the genus 
Eopleurotoma, of Cossmann, where the embryo is paucispiral 
and obtuse in normal European species and multispiral and 
more acute in others. The American representatives such as 
sayi, haeninghausi, nodocarinata and many more, have a 
multispiral, closely coiled and rather acute embryo of at 
least four whorls, although evidently typical Hopleurotoma 
otherwise and so regarded by Cossmann himself; the latter 
author, however, does not allude to the radical variations 
of the embryo. The second instance is that of Plewrofusia 
De Greg., the type of which is the American Oligocene ser- 
vaia of Conrad, containing many species having the embryo 
typically multispiral, conical, pointed and closely coiled, but in 
such forms as collaris and hilgardi Csy., of the Jacksonian 
Eocene and declivis Con., of the Vicksburg Oligocene, the 
embryo becomes obtuse and paucispiral, in the last named 
