PROSOBRANCHIATA. 205 
With regard to the genus Clavatula, several shells are found among the English 
eocene Pleurotome, which, agreeing with Lamarck’s definition of the genus, might be, 
perhaps correctly, referred to it; but since that genus, as re-defined, depends on 
zoological characters, and on the condition of the operculum, criteria which are not 
available to the palzeontologist, and the species themselves do not exhibit any characters 
by which they can be separated from the true P/leurotome, those shells have been 
referred, in the following descriptions, to the present genus. 
Bellardi, in his elaborate and most useful work, ‘Monografia delle Pleurotome 
fossile del Piémonte,’ has divided the Pleurotome into three genera, Plewrotoma, 
Borsonia, and Raphitoma. The first comprises the true Pleurotomee and the Clavatulee 
of Lamarck ; and with these are associated some fusiform shells generally referred to 
Fusus, the outer lips of which present, not the true notch or slit characteristic of a 
Pleurotoma, but a wide undulation, which the author regards as a “rudimentary sinus.” 
No other reason is assigned, and this certainly does not appear to be a sufficient one, 
for placing the shells in question in the present genus. Several of the so-called eocene 
Fusi present this undulation in the outer lip; and inasmuch as to refer them to the 
present genus would, in my opinion, uselessly create much confusion, I have left them 
among the Fusz, where they were first placed. The second genus, Borsonia, is proposed 
for certain shells in which the true sinus of a Pleurotoma is associated with a fold on 
the columella.* The remaining genus, Raphitoma, consists of those species in which 
the sinus is very small and confluent with the suture, and the canal is indistinct, a 
division which corresponds pretty accurately with MWangelia (Leach). The Pleurotcme 
are again divided into three sections, according to the size and shape of the sinus; 
namely, Pseudotomate, or false-notched shells, composed of the fusiform species before 
mentioned, in which the outer lip presents the so-called rudimentary sinus ; megafomate, 
or widely-notched shells; and macrotomate, or deeply-notched shells. The last 
section is again sub-divided into five groups; de/fordee, in which the canal is but little 
produced, and the sinus is placed in an angular depression ; pteroidee, in which the 
canal is elongated, the outer lip aliform and produced in front, and the sinus is in a 
depression ; carinifere, having the canal as long as the spire, and the sinus on a keel ; 
excavate, in which also the canal is as long as the spire, but the sinus is between the 
shoulder and the suture; and emicycloidales, having the canal indistinct, and the sinus 
semicircular, and placed in a depression. 
Although this classification will render great assistance in the study of the present 
* Shells referable to this division, as enlarged by Rouault, occur in our middle eocene strata; the 
genus Borsonza will therefore be noticed in its proper place. 
+ Bellardi cites Tomella, Swains., as corresponding with his section Megatomate ; that section, 
however, consists of two species only, P. cataphracta, Broc., and P. ramosa, Bast., in both of which the 
shells are many whorled, turreted, coronated, and concentrically striated, and therefore do not at all agree 
with Mr. Swainson’s definition of his genus T’omelia. 
27 
