316 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 (/oc. 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 Pleurotome 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. denticu/a, 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, | inch and 8-12ths; diameter, 8-12ths of an inch. 
Localities.—Brockenhurst, Lyndhurst, Roydon, and Whitecliff Bay (fide Forbes). 
