PROSOBRANCHIATA. 243 
bifurcatis ; ultimo anfractu brevi, ad basin tenuissime striato: aperturd ovali, in canali 
brevi, mediocriter lato, exeunti ; labro tenuissimo, intis plicato ; sinu lato, minime profundo, 
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 costelle, 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 costelle, 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. zanexa, 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 
costellz closely approximated instead of being separated by a wide, well defined 
furrow; the costelle themselves bifurcated, and the transverse lineation closer and 
less prominent than in P. ianeva 
It is difficult to distinguish P. dubia (Def.) for 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 Pleurotome, 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 cost, 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. inflewa is a subordinate character, is in the Crag shells strong and distinct ; 
