﻿PROSOBRANCHIATA. 



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Hill, the species is represented by the variety odontella; 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. 



In another variety, conidus, 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 Pleurotomse 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 Leognan, 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 longava 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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