Casey — Notes on the Pleurotomidae. 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 
SENeLAl lyst ALME LL yieretelaieye!s elausiel< ateleysietn\cisiaiotel sie alotaterstatnciciaietaie etelcten ce 
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........ Zs 
2— Anal sinus at least distinguishable and usually deep.... 
ANID SINUS) ODSOLELE Ye, cvefeiois} ilercie! slcjc\e!ofa's) [niesal:«tsla’e\ s eleveva siavel s) aici cistule teeiataucrctever erainte 
3— Sinus more or less distant from the suture........... 
Sinus adjacent to the suture....,......- seseee salete ele 
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 frequently wanting............0..0+0+++.++, P-LEURUTOMINI 
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 
PATE LIVING 02625000 ccecescccccevveene cscs secs se0e cesseeces e. BELINI 
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coum & 
