Casey — Notes on the Pleurotomidae. 129 



special study, either because of their relative importance or 

 in order to correct certain errors which have appeared in 

 print. 



Pleurotomini. 



This tribe is the most extensive of the operculate series and 

 comprises the largest and most conspicuous species of the 

 family. Excepting certain aberrant forms such as Anti- 

 planes and Genota, the latter of which was considered a Conicl 

 by Uossmann, though probably more correctly a Pleurotomid, 

 the genera arrange themselves naturally about the types 

 known as Pleurotoma, Surcida and Clavatula, the first being 

 distinguished by the entire absence of longitudinal ribbing 

 and corresponding prominence of the spiral sculpture, the 

 second having well developed ribs as a rule and inconspicuous 

 spirals, and the third — a special type — distinguished in gen- 

 eral by a spiniform modification of the ribbing. These three 

 type forms diverged from a common descent stem in very re- 

 mote time, the third being the most modern offshoot and 

 peculiarly developed in the middle and upper European Ter- 

 tiaries and in the living fauna of West Africa. In addition 

 to these, there are certain peculiar fossil types which did not 

 survive the world-wide revolution at the close of the Olioocene, 

 among which may be mentioned Scobinella with related 

 genera, distinguished by absence of true ribs, strong spirals 

 frequently interrupted by arrested growth giving a character- 

 istic tessellated sculpture and usually having strongly devel- 

 oped plicae on the columella, a character extremely rare or 

 perhaps entirely wanting among the living forms of the tribe. 



Pleurotoma Lamk. 



This genus is composed of large and rather slender species, 

 with a high and many whorled spire which is generally some- 

 what twisted apically, long and more or less contorted though 

 untwisted canal and a deep parallel-sided anal sinus, which is 

 not situated on the peripheral carina, but formed on a flat 

 depressed spiral band just behind the periphery. The sculp- 

 ture consists of broad close-set and obtuse spiral carinae, the 



