Casey — Notes on the Plexirotomidae. 125 



to be understood that the differences between these species 

 or genera are in any way equal, for data enabling us to esti- 

 mate these interrelationships are in almost all instances lack- 

 ing, and, in any event, they would be very difficult to decide, 

 for it frequently happens that species which appear to re- 

 semble each other very greatly may really be less affiliated 

 by bonds of true affinity than others which differ more 

 conspicuously. 



I would prefer therefore to divide a large family or subfamily 

 like the Pleurotomidae, into more or less definitely limited and 

 definable tribal groups, and to regard most of the subdivi- 

 sions under these tribal headings as true genera until their values 

 can be determined more accurately. This course would seem 

 to be preferable for example in the recently published classi- 

 fication of the Pyramidellidae by Dall and Bartsch (Proc^ 

 Biol . Soc . Wash . , XVII ) . The tribes in this case — Pyramid* 

 ellini, Turbonillini, Odostomiini and Murchisonellini — are 

 probably somewhat less differentiated than those of the 

 Pleurotomidae, but there have been so many generic names 

 published that this course would at least appear to be more 

 convenient than that followed. 



The eight tribes of Pleurotomidae indicated by material at 

 present accessible to me may by defined or outlined by the 

 following general characters : — 



Animal bearing an operculum; shell usually many-whorled ; outer lip 

 generally thin at maturity 2 



Animal non-operculate ; shell generally smaller and with fewer whorls, 

 frequently characterized by a greater development of plicae about the 

 contour of the aperture and a thickened outer lip at maturity 7 



2 — Anal sinus at least distinguishable and usually deep 3 



Anal sinus obsolete 6 



3 — Sinus more or less distant from the suture 4 



Sinus adjacent to the suture 5 



4 — Inner lip with little or no deposit of callus posteriorly; shell generally 



large and moderately thin in substance, the canal usually elongate, 



the ribbing f requ ently wanting Plburotomini 



Inner lip with thicker deposit of callus particularly evident posteriorly ; 

 shell smaller in size as a rule and thicker in substance, the canal 

 usually much abbreviated; ribs never obsolete Clavini 



5 — Sinus deep and distinct; shell generally very thin and fragile in sub- 



stance, inhabiting palaearctic waters exclusively and for the most 

 part living Bklini 



