128 Trans. Acad. Sci. of St. Louis. 
species indeed so notably obtuse as to appear swollen owing 
to the very large nucleus or posterior tip of the embryo — fol- 
lowing the terminology of Cossmann. It appears unwarrant- 
able to separate these paucispiral forms from the others, as in 
‘ general habitus, type of sculpture and all other features they 
seem to be perfectly congeneric. The third genus, which was 
founded by De Gregorio upon the American Oligocene 
cochlearis Con. and named Pleuroliria, has a multispiral 
embryo with ribbed lower whorls in its earlier geological 
stages, but a very small and obtuse paucispiral protoconch, 
with larger nucleus and almost entirely obsolete riblets, 
in its later history, the general characters of the shell 
remaining constant throughout to such an extent that the 
living form has recently been declared identical with the 
Oligocene type;—to a superficial view they are indeed 
completely identical, but the embryos are strikingly differ- 
ent. These examples are mentioned to indicate in some 
measure the great difficulties and uncertainties that will 
attend the rigorous definition of genera in the Pleurotomidae — 
difficulties which are accentuated among the multitude of 
small non-operculate forms, which, being comparatively mod- 
ern in development, have the intermediate gradations not 
yet so thoroughly eliminated as is the case with the almost 
equally numerous but geologically much older operculate 
series. 
While I feel certain that the number of true genera of 
Pleurotomidae is much greater than usually admitted, the 
above discussion may serve to indicate some of the obstacles 
which will be encountered in the conscientious study of them, 
obstacles which will not be removed until we understand more 
thoroughly than now the meaning of all the modifications of 
structure and sculpture displayed so lavishly among these 
wonderful and fascinating objects. These modifications of 
sculpture, form and coloration all mean something, and are 
caused by varying conditions of adaptation to environment 
in its broadest sense. 
In the subsequent discussion of the tribal groups outlined 
above, I have included a few genera which seem worthy of 
