26 MOLLUSCA FROM THE GREAT OOLITE. 
truncated, the aperture subquadrate, acute above, widely notched at the base, but not 
deeply nor recurved ; columella curved, and turning inwards at its base, which is poited ; 
it is rounded and smooth ; the inner lip is effuse, rather depressed in the middle, covering 
an umbilicus ; the outer lip is thin and somewhat sinuated. : 
This is one of the most remarkable of the Great Oolite genera of Univalves, and has 
not as yet been found in any other than the oolitic rocks. It constitutes an addition to the 
Purpurifera of Wamarck, or the Eutomostomata of De Blainville. The following characters 
in their combination will be found sufficiently to distinguish it from all other known genera = 
the truncated base, the wide and shallow notch, the columella smooth, rounded, and curving 
inwards, the concealed umbilicus, and the thin sinuated outer lip. The young shells are 
delicately striated or grooved, the basal notch is scarcely formed, and they are perfectly free 
from adherent shells. On the other hand, the full-grown shells are always more rugose ; 
with advance of age their sulcations or other markings become irregular; or are nearly 
obliterated, the basal notch becomes more important, and not unfrequently the whole’ 
external surface becomes covered with adherent shells. It would even seem that those 
encrusting shells were carried about by the animal during life. They are never found upon 
the young shells, or within the aperture, upon the left lip, about the basal notch, or, in’ 
fact, upon any part which was in contact with the soft parts of the animal. As the 
Purpuroidea are found lying in every possible position, the absence of adherent shells upon 
the parts in question may be held conclusive as to their period of attachment. 
It will be seen, then, that the generic characters above enumerated acquire importance 
only upon their bemg viewed im combination. Owing, perhaps, to a want of attention to 
this circumstance, it may be that an undue value has been assigned to one or two cha- 
racters, or to the inspection of ill-preserved specimens, or the want of a sufficient number 
to exhibit their several phases of form and markings ;—to one or all of these causes of error 
we may ascribe the fact, that one of our species has already been thrice figured and 
described under two generic and three specific designations. 
The beds of planking upon Minchinhampton Common are the productive site of this 
genus. ‘The shells are clustered together over a small area. Originally the space was 
about 100 yards in length and half that extent in breadth ; but from the rapid quarrying 
of the stone, which there occurs in very large blocks, by far the greater portion is now 
removed, and the genus has already become comparatively scarce. ‘I'wo other localities, 
near and upon the same geological position, have furnished it, but very rarely, and in a bad 
state of preservation. In the upper division of the Great Oolite near Minchinhampton 
(from the white limestone upwards), the genus is likewise found occasionally over small areas, 
and in considerable numbers; but, owing to the compactness of the investing limestone, 
the shells can never be extricated except as casts. In this condition, with some small por- 
tion of the shell preserved, they resemble the specimen figured in the ‘ Mineral Conchology,’ 
t. 578, fig. 4; but when entirely denuded of the crystalline shell, they have the aspect of 
Natica, and without great care might be taken for that genus, the surface is smooth, 
