PROSOBRANCHIATA. 293 
each extremity: these plications, which vary much in different individuals and 
frequently in the same specimen, become gradually less prominent as the shell 
approaches maturity, and are altogether lost on the last whorl. The posterior 
margins of the whorls are concave and bordered round the suture by two fine, thread- 
like, raised lines, frequently granulated by the lines of growth. The concentric lines 
over the posterior margins and shoulders of the whorls are very fine, closely set, equal, 
and but slightly raised ; over the middle and front parts of the whorl they are distant, 
irregular, elevated, occasionally faintly granulated, and separated by broad, flat spaces, 
not infrequently traversed by single, very slender lines. The aperture is of an 
oblong-ovate form, and terminates in front in a wide, but short and indistinct, canal. 
The outer lip is much arched, rather wing-shaped, thin and sharp on the edge, 
and smooth and simple within; and the sinus, which is placed on the shoulder, is 
wide, very deep, somewhat triangular in form, with a broad extremity. 
This Pleurotoma presents a remarkably close analogy with P. wniserialis (Desh.), to 
which species, in fact, it has generally been referred. It is, however, a wider and 
coarser shell, with a blunter spire, and the plications are more vertical, more irregular, 
and not so crescent-shaped as in the French shells; these variations, joined with the 
different condition of the sinus, which, in the French shell, is described as narrow, 
not deep, and with parallel margins, apparently justify a separation of the two 
shells. 
Size.—Axis, one inch and 4-12ths; diameter, rather more than 5-12ths of an inch. 
Localities—Middle Hocene ; Barton, Highcliff. London Clay, Clarendon. 
No, 217. Pirzvroroma vagians. J. £. Edwards. Tab. XXXI, fig. 12, a, 3. 
P. testa elongata, turritd, omnino concentrice lineatd: anfractibus sub-convexis ; ad 
humeros obtuse angulatis, plicato-crenatis ; antice repente coarctatis; marginibus posticis 
concavis, ad suturam marginatis ; plicis angustis, cuneiformibus, obliquis ; lineis concentricis 
posticis numerosis, equalibus ; lineis medianis distantioribus, prominentioribus, granulatis : 
apertura ovali, in canali patulo, brevi, exeunti ; labro leviter arcuato, acuto, intis levi; sinu 
ad humerum collocato sub-profundo, triangulari. 
Shell elongated, turreted, concentrically lined ; the spire, formed of six or seven 
volutions, is rather thick, pointed, almost mucronate, at the extremity, and much 
produced, equalling 3-5ths of the entire shell in length. The whorls are slightly 
convex, bluntly angulated at the shoulders, and ornamented with a single row of 
narrow, oblique, wedge-shaped crenulations, rather closely set in the early whorls, but 
becoming more distant on the penultimate and last whorls; the posterior margins 
are deeply furrowed and thickened round the suture, where they present a sharp, 
ridge-like iine, frequently crenulated. The concentric lines over the posterior margins 
