﻿PROSOBRANCHIATA. 



265 



exceeding the aperture in length. The whorls are slightly convex ; tire posterior 

 margins somewhat depressed and concave, giving a turriculate aspect to the spire ; a 

 single row of roundish or slightly oblique, oblong granulations runs round the sutural 

 margin, the space between which and the shoulder is nearly equally divided by a 

 raised line granulated by the prolongation of the costelke over the margins ; the 

 costellae, which are more or less curved in different individuals, extend a little beyond 

 the middle of the whorls, and gradually attenuate as they recede from the shoulders ; 

 the concentric lines are prominent, numerous, and irregular ; the two lines on the 

 shoulder and on the margin being more elevated than the rest. The aperture is of an 

 oblong, oval shape, and terminates in front in a short, moderately wide canal ; the 

 outer lip is thin, sharp on the edge, and wing-shaped; and the sinus, which is wide 

 and nearly semicircular, occupies the whole breadth of the margin. 



This Pleurotoma is one of the shells figured under the superintendence of Mr. 

 Charlesworth, for the British Natural History Society, and the elegant ornamentation 

 which characterises it, well merits the epithet selected for a specific name. The shell, 

 however, in its general form and the character of the ornamentation, so closely 

 resembles P. granulata (Lamk.), that it is difficult, if not impossible, satisfactorily to 

 separate the two. It will be found, on comparison, that the English shell is broader, 

 the spire not so pointed, and proportionately shorter, and the aperture, consequently, 

 relatively longer than in the French shell. These distinctions may fairly be attributed 

 to external conditions, and I therefore regard the English shell not as a separate 

 species, but merely as a strongly marked local variety of P. granulata. 



Size. — Axis, 5-12ths of an inch nearly (10 millim.); diameter, 2-12ths of an inch 

 (4 millim.) 



Localities. — Barton, Alum Bay (Stratum No. 29, Prestwich) ; Highcliff, where it 

 occurs in such profusion that it may be regarded as one of the characteristic shells of 

 that deposit ; and Bracklesham Bay. French: Grignon, Parnes, Mouchy, Courtagnon 

 (fide Desk.), Monneville, Varinfroy (fide Graves). 



No. 187. Pleurotoma Headonensis. F.E.Edwards. Tab. XXVIII, fig. 17, a — c. 



Pleurotoma Headonensis. (Edw.MSS.) Morris. 1856. Forbes's Tert. Fluv. Mar. Format. 



&c; Mem. Geol. Surv., p. 155, t. 5, fig. 4. 



P. testa gracili, fusiformi, longitudinaliter semi-costellatd, spiraliter sulcata: spird 

 elevatd, longitudine dimidium totius testa paulo superanti: anfractibus convexiusculis, 

 postice unico sulco prof undo exaratis, ad suturam marginatis ; costellis numerosis, per- 

 brevibus, anguslis, obliquis ; sulcis spirallbus regularibus, sub-distantibus : aperturd elongato- 

 ovali, in canali latiusculo, brevi, exeunii ; labro leviter arcuato ; sinu sub-triangulari, lato, 

 minime prof undo. 



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