NOTES ON THE PLEUROTOMIDAE WITH DESCRIP- 

 TION OF SOME NEW GENERA AND SPECIES. 



Thos. L. Casey. 



In the " Structural and Systematic Conchology " of Try on, 

 the family Pleurotomidae is restricted to two genera — Pleu- 

 rotoma and Holla. The latter of these being somewhat doubt- 

 ful in its relationship, we may assume that in the original 

 opinion of Tryon the family Pleurotomidae is made up of the 

 single genus Pleurotoma, which he separated into a number of 

 named groups relegated to subgeneric rank. Now while per-'' 

 fectly true that, with our present lights, well-nigh insuperable 

 difficulties beset us in striving to define isolated aggregates of 

 species and classify them in a satisfactory manner, it none 

 the less appears to the writer that the course suggested by 

 Tryon was unphilosophic. Although there seems to be 

 scarcely any limit to the number of genera that might be pro- 

 visionally admitted — a recent study of rather more than 

 600 species living and fossil, most of which are in my 

 cabinet, apparently indicating nearly or quite 250 genera, 

 based principally upon embryonal structure, form and 

 position of the anal sinus, form and extent of the 

 canal and general type of sculpture — that is no adequate 

 reason for holding all this diversification of structure and 

 evidence of adaptation to greatly varied environments among 

 at least 3000 species, from the Cretaceous to the present 

 time, to be comprised within a single genus. Later, in the 

 "Manual of Conchology," Tryon considerably modified 

 his original views and admitted several distinct genera 

 which he assigned to sundry groups held to have subfamily 

 rank, but, in this case, it would appear that he rather over- 

 estimated the taxonomic importance of certain genera. 

 Granting that the family under consideration comprises 



only the two genera Pleurotoma and Halia, as originally 



(123) 



