PROSOBRANCHIATA. 289 
Hill, the species is represented by the variety odontel/a; in this the shell is more 
slender and shorter than in the typical form; the posterior margins are smooth 
or very feebly lined, and the tubercles are smaller and more pointed. 
Besides these varieties, other forms occur which apparently are confined to the 
middle Eocene deposits. In the first of these, constituting the variety mutica, from 
Highcliff, the spire is obtuse, the concentric lines over the posterior margins and 
in the spaces between the prominent lines in front are crowded and so fine as 
to be barely visible to the naked eye; and the tubercles are very small, close-set, and 
frequently obsolete on the later whorls, which then present a simple, keel-like line on 
the shoulders. 
Tn another variety, conw/us, from Highcliff and Barton, the shell, as in the varieties 
gracilenta and odontella, is smaller and more slender than in the type; the spire is 
more produced, the margins of the whorls are nearly straight, imparting a 
conical character to the spire; the concentric lines over the margins are sharp, 
regular, and decussated by the prominent lines of growth, while those over the 
middle of the whorls are thin, elevated, distant, and simple; the tubercles are 
small and compressed, frequently assuming a tooth-like appearance; and the an- 
terior canal is short and somewhat oblique. In all these varieties, however, the 
essential specific characters are preserved. 
This species forms one of a group of Pleurotome which present a very 
striking similarity in their general aspect and ornamentation; they are all 
distinguished by the lengthened spire, the prominent transverse lineation, and 
the obtuse, tuberculated carina on the shoulders of the whorls, caused by the 
successive thickened extremities of the labial sinus. To this group belong P. 
(Murex) monilis (Brocchi) and P. trifasciata (Bellardi); species which appear to be 
separable from the present, not so much by differences in the transverse 
lineation or the condition of the carina, as by the greater width of the shells and 
the shorter and more cup-like form of the whorls. These peculiarities are parti- 
cularly noticeable in the figure of P. denticula, as figured by Basterot. I have 
not been able to procure any well-authenticated Bordeaux specimens of Basterot’s 
species; but the typical form of the shells described by Sowerby as P. plebeia so 
closely resembles not only specimens from Léognan, referred to P. denticula, with 
which I have compared them, but also those from Tortona and the environs of Turin 
referred to the latter species by Bellardi, while the varieties /ongeva and macrobia 
agree so well with the shells from the Bolderberg recorded by Nyst, and those 
from Pau described by Rouault, that in my opinion the English shells cannot be 
satisfactorily regarded as specifically distinct. The differences will be found to lie 
chiefly in the transverse lineation, the condition of the tubercles, or the internal 
plication of the outer lip. Now, the transverse lineation is a very variable character 
in the present species, frequently differing in specimens from the same locality ; the 
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