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 PleuroliiHa, 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 senera 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 



