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. {=Hemipleiirotoma 

 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 sev- 

 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 Eopleurotoma 

 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 JPleurofusia 

 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 o btuse and paucispiral, in the last named 



