PROSOBRANCHIATA. 205 



With regard to the genus Clavatula, several shells are found among the English 

 eocene Pleurotoma, 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 palaeontologist, and the species themselves do not exhibit any characters 

 by which they can be separated from the true Pleurotoma, 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 Piemonte,' has divided the Pleurotomge into three genera, Pleurotoma, 

 Borsonia, and Raphitoma. The first comprises the true Pleurotomge and the Clavatulae 

 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 Fusi, 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 Mangelia (Leach). The Pleurotoma 

 are again divided into three sections, according to the size and shape of the sinus ; 

 namely, Pseudotomata, or false-notched shells, composed of the fusiform species before 

 mentioned, in which the outer lip presents the so-called rudimentary sinus ; megatomata, 

 or widely-notched shells ;f and macrotomata, or deeply-notched shells. The last 

 section is again sub-divided into five groups ; deltoidea, in which the canal is but little 

 produced, and the sinus is placed in an angular depression ; pteroidea, in which the 

 canal is elongated, the outer lip aliform and produced in front, and the sinus is in a 

 depression ; carinifera, having the canal as long as the spire, and the sinus on a keel ; 

 excavates, in which also the canal is as long as the spire, but the sinus is between the 

 shoulder and the suture ; and hemicycloidales, 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 Borsonia will therefore be noticed in its proper place. 



f Bellardi cites Tomella, Swains., as corresponding with his section Megatoviatce ; that section, 

 however, consists of two species only, P. cataphracla, 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 Tomella. 



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