EXPLANATION OF THE PARTS 01 fciHELLS. 19 



ter is most commonly to be met with among species of Buccinum 

 and Murex. 



Shells which have no umbilicus are termed imperforate. 



SipJmncuhis is that small round perforation which forms a com- 

 munication between the chambers of the Spirulae, and penetrates 

 through the whole spire of the shell. Plate III. fig. 11, v. 



Varices are transverse ribs which cross the whorls of shells in 

 some species of Buccinumj- 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 III. fig. 12, R R R R. 



Teeth of univalves, or tooth-shaped ( rotuberances, are fine, 

 white laminse, or ridges, running spirally backvvards, 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 cuticle 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 in a state of nature. 



