A CRETACEOUS GASTROPODA 
A shell, which is coiled in a regular plane, is umbilicated on one or, generally, 
on both sides; but in most cases, the basal (or lower) is deeper than the apical 
(or upper) excavation. When the volutions are coiled in such a manner, as to 
rest on the upper part of the succeeding whorl with their whole basis, or only with 
a portion of it, while the apex of the shell is raised conically, a similar conical 
excavation is thus formed on the side opposite to the apex, or internally of the 
shell. In this case we call the excavation an ‘umbilicus,’ the form and size of 
which depends upon those of the shell. The central perpendicular axis of umbili- 
cated shells, which expresses their height, is, therefore, only imaginary. Further 
it depends on the angular or rounded forms of the whorls, and on the kind of invo- 
lution or coiling, whether the inner volutions are partially visible within the 
umbilicus, or whether this latter forms a regular hollow cone, as in many Helicide, 
on the one hand, and in many Zrochide on the other. The umbilicus may often 
be covered with a callose mass, which on the further growth of the shell may be 
absorbed, or may remain and fill up the excavation. Instances of this kind occur 
in the Naticide, Neritopside and others. 
When, on the contrary, the basal portion of each whorl is partially covered or 
embraced by the succeeding whorl in order to form a certain axis, round which 
the volutions are coiled up, we call this axis a columella. The formation of this 
columella depends on whether the inner lip covers the preceding whorls in its 
entire extent, or only partially, and whether it is very thin or thickened; according 
to this the columella is either solid or hollow, and in itself either very thin and 
loose, or strongly and consistently built by the callose mass of the inner portion of 
the peristome. Solid columelle are found in Fusus, Buccinum, ete., hollow in 
Cancellaria, Nerinea ; very thin and slight in Twrritella, strong in Rostellaria and 
Cerithiwn, ete. 
The inner lip sometimes remains partially free at the base, so as to forma 
hollow in the columella. In growing larger, however, the free or raised portion of 
the inner lip is absorbed and, while forming a solid columella, the same hollow is 
again restored at the new peristome of the aperture, as is, for instance, observed in 
some species of Chemnitzia and others. In this case, the expression jfisswe may 
probably be well appropriated. Depending on the peculiar character of the spiral 
coiling of a shell, it will be seen, that while in some cases there appears to exist a 
very sharp distinction between umbilicus and columella, in others they approach, 
or even amalgamate with each other. Taking it very strictly we may say, that a 
columella is always present, when the singular whorls are contiguous and coiled 
cone-like. In shells, the whorls of which are not contiguous, as is sometimes the 
case in Scalaria, Vermetus and others, we can speak neither of a columella, nor of 
an umbilicus. é 
Proceeding with the descriptions of the species, we shall often note a few 
measurements, as— 
1.—tThe spiral angle, according to D’Orbigny’s Helicometer ; 
2.—The sutural angle, by which we understand the angle formed by the 
ascending suture and a horizontal plane at right angles to the axis of the shell. 
