EXPLANATION OF THE PARTS OF SHELLS. 19 
ter is most commonly to be met with among species of Buccinum 
and Murex. 
Shells which have no umbilicus are termed imperforate. 
Siphunculus is that small round perforation which forms a com- 
munication between the chambers of the Spirule, and penetrates 
through the whole spire of the shell. Plate II. fig. 11, v. 
Varices are transverse ribs which cross the whorls of shells in 
some species of Buccinum, Murex, and Tritonia, and exemplified 
in the Cassidaria. Varices are formed by the periodical growth 
of the shells, these being the margin of the outer lip, to which the 
animal has attached its periodical enlargements. In some species 
they have more the form of sutures than ribs; this is owing to 
the margin of the outer lip being but slightly developed. 
Ribs are those longitudinal and transverse protuberances which 
are in many of the univalve shells. Plate II. fig. 12, RRRR. 
Teeth of univalves, or tooth-shaped y;rotuberances, are fine, 
white lamin, or ridges, running spirally backwards, in a parallel 
direction to each other; those on the exterior lip may, in most 
instances, be traced through the outside of the shell, and are 
nearly alike in length. Plate III. fig. 13, a. 
Epidermis is a skin, or cuticle, covering the exterior surface of 
shells, destined by nature to protect their surface from being in- 
jured. It is membranaceous, somewhat similar to the periosteum 
which covers the bones of animals. This substance is the pro- 
duction of the animal inhabiting the shell ; it is uniformly observed 
in some species, and not at all in others. Shells with a rugged 
or uneven surface have almost always this epidermis. In some 
it is strong, laminated, velvety, fibrous, or rough, often beset with 
long hairs, and in others very thin, smooth, and pellucid, and 
admits the colours of the shell to shine through it. In some 
species it is so dusky that it entirely obscures the beautiful colour- 
ing of the shell beneath. Although many shells are very beauti- 
ful, even with this cvticle on them, they are much more so when 
it is removed; but I would by no means advise the collector of 
shells to remove it, unless he may have several duplicates of the 
same species; and then he may do it by way of variety. It is 
always preferable, however, to keep them ina state of nature. 
