153 
HELICINA polita. 
TAB, CCLXXXV. 
Spec. Cuar. Subdiscoid, polished ; spire elevated, 
acute; volutions marked with an impressed 
band, depressed above, below ventricose ; 
callus thin, expanded ; aperture nearly square. 
ee 
"Tue short conical spire of this shell is rather concave 
on the sides, in consequence of the upper parts of the 
whorls being rather hollow: the lower part of the last 
whorl is separated from the spire by an impressed band, 
upon which the lines of growth are seen arched the 
reverse way, indicating a sinus in the right lip; the left 
lip of the mouth is reflected a considerable way over the 
base of the columella, forming in young shells a promi- 
nent callus, but in old ones it is pressed beneath the 
surface of the shell, leaving the columella, however, 
thickened. The height is about two-thirds the diameter. 
So variable are the characters of the shells possessing 
the sinus in the lip, indicated by the band around the 
whorl, some possessing those of Trochus, others those 
of Helicina, as the present, and H.expansa, t. 273, &e. 
that we can hardly assign limits to the genus Pleuroto- 
maria; it might even be shewn to be necessary to form 
two genere. 
The shell before us is from the sandy stone between 
the Lyas clay and the iron-shot Oolite : it was discovered 
by the Rev. W. D. Conybeare, in the neighbourhood of 
Cropredy. 3 
