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 Anfi- 
planes and Genota, the latter of which was considered a Conid 
by Cossmann, though probably more correctly a Pleurotomid, 
the genera arrange themselves naturally about the types 
known as Pleurotoma, Surcula 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 Oligocene, 
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 
