﻿PROSOBRANCHIATA. 



243 



bifurcatis ; ultimo anfractu brevi, ad basin tenuissime striato : aperturd ovali, in canali 

 brevi, mediocriter lato, exeunti ; labro tenuissimo, intus plicato ; sinu lato, minime prof undo, 

 sub-semicirculari, in margine collocato. 



Shell elongated, narrow, ribbed, and spirally lined : spire elevated, formed of eight 

 or nine volutions exclusive of the pullus: whorls slightly convex, short, and 

 thickened at the sutural margin, which is ornamented with a single row of bold, 

 roundish or oblong beads, imparting somewhat of a turreted character to the spire. 

 The surface of the whorls presents numerous costellse, long, narrow, curved, swelled 

 at the top, and corresponding with the marginal granulations, so as to resemble 

 a semicolon, from which circumstance the name given by Sowerby was taken ; 

 the costellse, as they pass over the centre of the whorl, bifurcate and disappear 

 towards the base of the shell : the concentric lines are not very prominent, but 

 are most conspicuous on the anterior part of the shell, becoming feeble as they ascend 

 the whorl : they become almost obsolete in the adult shell. The aperture is ovate, 

 one third of the whole length of the shell, and terminates in a short, moderately 

 wide canal ; the outer lip is slightly arched, very thin, and plicated within ; the 

 sinus is wide, shallow, nearly semicircular, and placed in the margin. 



This species presents a very close resemblance to P. innexa, of which many 

 may perhaps be inclined to regard it as a local variety. The shell, however, is 

 narrower ; the spire more slender ; the sutural granulations and the extremities of the 

 costellse closely approximated instead of being separated by a wide, well defined 

 furrow; the costellse themselves bifurcated, and the transverse lineation closer and 

 less prominent than in P. innexa. 



It is difficult to distinguish P. dubia (Def.) from the present species. In Defrance's 

 species the longitudinal ribs are fewer and rounder ; but the ornamentation of the two 

 appears to be similar in the essential characters, and I should have regarded the two 

 species as identical. M. Deshayes, however, with the advantage of an extended 

 comparison, has considered P. dubia as distinct, an opinion in which both Bronn 

 and D'Orbigny concur, and I have therefore cited Defrance's species with a doubt 

 as to the identification. 



Nyst has referred some shells from Vliermael to P. semicolon (Sow.), which are 

 described as granulated, angulated at the shoulders, and concentrically striated, 

 but not as being costellated ; these shells cannot, therefore,, be considered as correctly 

 identified. 



The Crag Pleurotomse, referred by Mr. S. Wood, doubtingly, to this species, 

 appear to be distinct; they are both broader shells, with angulated and strongly 

 tuberculated whorls, the posterior margins of which are wide and broadly concave ; 

 and the costse, which in the present species form a prominent character, appear 

 to be obscure or wanting ; while, on the other hand, the transverse lineation, which 

 in P. inflexa is a subordinate character, is in the Crag shells strong and distinct ; 



