234 EOCENE MOLLUSCA. 



Var. 6th. pagoda; fig. 10/^. Testa latiori, fasciolis et lineis spiralibus elevatis 

 cinctd: anfractibus bicarinatis, ad suturam marainatis; carinis simplicibus, singulis 

 unico sidco exaratis ; carina posticd superne revolutd ; fasciolis numerosis, inaqualibus, 

 lavibus. 



The typical form of this elegant Pleurotoma is distinguished by the smooth concave 

 margins of the whorls, by the remarkable regularity of the denticulation on the keel, 

 resembling the teeth of a circular saw, and by the close-set rows of even, bead-like 

 granulations, which cover the middle and front parts of the shell, slightly dimi- 

 nishing in size as they approach the anterior extremity of the canal. The shell, upon 

 which Lamarck formed the species, was found at Parnes; and, apparently, the orna- 

 mentation which characterises it is confined in the French specimens to those from 

 the "calcaire grossier." The species occurs, also, in the older eocene formations of that 

 country ; but the shells from them are generally of smaller dimensions, and present 

 more or less strongly marked deviations from the typical ornamentation. In England 

 the species has not as yet been found above the older eocene deposits ; several forms, 

 indeed, occur in them ; one or two of which apparently agree with French varieties ; 

 but neither among them, nor, as I believe, among those from the " groupes glauco- 

 nieux" of France, does the exact ornamentation of the typical P. terebralis of Lamarck 

 occur. 



In the first variety (ditropis), which corresponds with specimens in my cabinet from 

 Cuisse-Lamotte, the shell is much smaller than that of the type, elongated, fusiform, 

 narrow, turreted, and beautifully ornamented with concentric rows of regular bead- 

 like granulations ; the spire is pointed and much produced, being rather more than 

 two fifths of the length of the whole shell. The whorls are somewhat convex, sharply 

 angulated, and carinated at the shoulders ; the posterior margins rather depressed, 

 slightly channeled, and perfectly smooth, except round the suture, where they are 

 bordered by a single row of small regular granulations ; occasionally a specimen occurs 

 in which the smoothness of the surface is broken by very fine curved plications formed 

 by the successive margins of the sinus. The keel on the shoulder is sharp, prominent, 

 and denticulated with great regularity : in front of this, just where the whorl begins 

 to contract, is a thick concentric band, rather obliquely plicated, and so prominent as 

 to present the appearance of a second keel on the whorl ; the ends of the plications 

 project beyond the edges of the band, giving to it a denticulated appearance. The 

 concentric rows are unequal, irregular, and more or less distant in different specimens, 

 but closer and more regular towards the anterior extremity. The last whorl is much 

 attenuated in front, and terminates in a moderately long canal : the sinus is rounded, 

 deep, and wide, occupying almost the entire width of the margin. 



In the next variety (concinna) the shell is somewhat larger than that of the preceding 

 variety, but yet smaller than that of the type ; the front keel is narrower and less 

 prominent than in the variety ditropis, and is evenly granulated ; the concentric rows 



