﻿PROSOBRANCHIATA, 



259 



angulated at the shoulders, to which the last but one of the concentric lines, more 

 prominent than the others, gives the appearance of being carinated. By this keel-like 

 line, the whorl is nearly equally divided ; the anterior half, covered with the concentric 

 lines, is fiat-sided; the posterior half, forming the margin of the whorl, is very 

 slightly channelled, almost straight, and smooth, except where it presents obscure 

 curved lines, indicating the earlier outlines of the sinus, and is bordered round the 

 suture by a single coarsish, thread-like, raised line. The posterior margins slope 

 backwards, at an angle with the shoulders corresponding with that formed by the 

 anterior portion of the whorl, giving a regular zig-zag outline to the spire. The ribs 

 are moderately distant, not very broad, rounded, and short, barely extending to the 

 middle of the whorls ; the concentric lines are thread-like, rather thick, and separated 

 by concave furrows as wide as the lines ; excepting the one on the shoulders, they 

 are nearly equal and regular. The aperture, which is of a widish-oval shape, 

 terminates in front in a short, slightly oblique, and not very wide canal, the anterior 

 extremity of which is a little bent backwards ; the outer lip, as indicated by the lines 

 of growth, is arched ; and the sinus, which extends over the whole width of the 

 margin, is deep and subtrigonal in shape. 



The wide, straight margins, angular shoulders, and zig-zag outline of the 

 whorls, which characterise this species, are not noticed in the description, nor 

 represented in the figure given in the ' Mineral Conchology.' The present Pleurotoma, 

 as described and figured in that work, would appear to resemble a well-known 

 Miocene species {Murex obhngus, Brocchi, the Pleurotoma dubia, Jan. ; P. obeliscus, 

 Des Moul. ; and P. midtinoda, Grat.) ; and this, probably, has misled Bellardi and 

 other continental authors into referring the Miocene shells to P. brevirostrum. In 

 these shells, however, the posterior margins of the whorls are narrow and concave, 

 the sides nearly parallel with the axis, the ribs long, extending to the front of the 

 whorls, and the anterior canal is very short and wide. P. brevirostrum, in fact, 

 more nearly resembles P. Lamarcki, Bell (P. semistriata of Partsch), a species 

 described by Homes as occurring in the Tertiary Beds of Germany ; and it agrees so 

 closely with some Miocene shells, from the neighbourhood of Vienna, in the British 

 Museum, that the latter cannot be regarded as specifically distinct. These shells 

 have been, I think incorrectly, referred to P. oblonga; they are smaller and narrower 

 than P. Lamarcki, and the margins of the whorls are not quite so concave, but they 

 may be, probably, a variety of that species. 



The shell on which the present species was founded, and for the use of which I am 

 indebted to Mr. Sowerby, was presented to the late Mr. Sowerby by Lady Burgoyne, 

 by whom it was stated to have been found at Muddiford ; I do not know of any 

 other specimen having hitherto been found, although the beds at Muddiford, High- 

 cliff, and Barton have, probably, been explored more thoroughly than any other 

 Eocene deposit in England. The shell in question does not present the aspect of 



