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 flat-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.’ he present Pleurotoma, 
as described and figured in that work, would appear to resemble a well-known 
Miocene species (Murex oblongus, Brocchi, the Pleurotoma dubia, Jan.; P. obeliscus, 
Des Moul.; and P. multinoda, Grat.); and this, probably, has misled Bellardi and 
other continental authors into referring the Miocene shells to P. drevirostrum. 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. Lamarchki, Bell (P. semistriata of Partsch), a species 
described by Hérnes 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; Ido 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 
