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EOCENE MOLLUSCA. 



keel, bearing a row of moderately distant, nodulous tubercles ; the posterior margins 

 are depressed, concave, bordered round the suture by two sharp, elevated lines, crenu- 

 lated by the lines of growth ; the hollow space between these lines and the shoulder is 

 traversed by numerous very slender lines, so fine as scarcely to be visible without the 

 aid of a magnifying glass or to detract from the otherwise smooth aspect of the sur- 

 face ; the concentric lines over the middle of the whorls are very prominent, thick, 

 cord-like, rather distant, and irregular ; two or three fine, thread-like lines very 

 often appear in the intervening spaces. The last whorl is much contracted towards 

 the front, and terminates in a short, distinct, and very wide canal, rather deeply 

 notched at the anterior extremity. The aperture is of an elongated, oval form ; the 

 outer lip much arched, thin and sharp on the edge, and smooth within ; the sinus, 

 which is placed on the keel, is very wide, moderately deep, and triangular in shape ; and 

 the columella is slightly twisted, and presents about the middle a very obscure callus, 

 and at the anterior extremity the crest, which usually accompanies a well-defined, 

 anterior notch. 



This Pleurotoma is, as Professor E. Forbes {Joe. cit.) has observed, much thicker, 

 wider, and larger than P. plebeia {denticula), of which, nevertheless, from an 

 assumed identity of sculpture in all essential points in both shells, that author con- 

 sidered it to be merely a variety. In this opinion I cannot concur. Without attaching 

 too much weight to the great differences in the size and relative proportions of the 

 two shells, although, when associated with other distinctions, these are not without 

 importance, it will be seen that, in fact, the sculpture is not identical with that of 

 P. denticula, and that there are other dissimilarities sufficient to separate the present 

 Pleurotoma from that species. With regard to the crenulation on the shoulders of the 

 whorls, that character is due, as before observed, to the thickening of the shell at the 

 extremity of the sinus, and a greater or less similarity in that ornament must necessarily 

 prevail in all the species forming the group to which the Pleurotoma? in question belong ; 

 but in this species, the crenulations are more transversely oblong and nodulous than 

 those which characterise the upper Eocene forms of P. denticula ; the spire also is more 

 pointed and shorter, the posterior margins more depressed, the transverse lineation 

 much more coarse and prominent, and the anterior canal shorter and wider, and deeply 

 notched at the extremity. On these grounds, therefore, I have separated the present 

 species from P. denticula, although I have much hesitation in dissenting from the 

 opinion of Professor E. Forbes. I may add that I possess a series of each form 

 from the same locality, Lyndhurst, in which the distinguishing characters of the two 

 species are constantly maintained, without the occurrence of any intermediate form. 



Size. — Axis, 1 inch and 8-12ths; diameter, 8-12ths of an inch. 



Localities. — Brockenhurst, Lyndhurst, Roydon, and Whitecliff Bay (fide Forbes). 



