234 EOCENE MOLLUSCA. 
Var. 6th. pacopa; fig. 10%. Zestd latiori, fasciolis et lineis spiralibus elevatis 
cinctd: anfractibus bicarinatis, ad suturam marginatis; carinis simplicibus, singulis 
unico sulco exaratis; carind posticd superne revolutd ; fasciolis numerosis, inequalibus, 
levibus. 
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 
‘uisse-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 
