EXPLANATION OF THE PARTS OF SHELLS. 31 



In some, almost entirely closes the opening or mouth. This 

 character is most commonly to be met with among species 

 of Buccinum and Murex. 



Shells which have no umbilicus are termed imperforate. 



Siphuncuhis is that small round perforation which forms 

 a communication between the chambers of the Nautili, 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 Buccinum, Murex, and Tritonia; 

 as exemplified in the Cassis Areola. Plate XI. fig. 21. 

 Varices are formed by the periodical growth of the shells, 

 they being the thickened 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 or transverse protuberances 

 which are seen in many of the univalve shells. Plate III. 

 fig. 12, r rr r. 



Teeth of univalves, according to Colonel Montagu, are 

 not properly denticles, or tooth-shaped protuberances, but 

 are fine white laminae, 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. tig. 13, a. 



Epidermis is a kind of cuticle, or membrane, covering the 

 exterior surface of many shells of all the orders, destined by 

 nature to protect their surface from being injured. This 

 substance is the production of the animal inhabiting the 

 shell; it is always present in certain species, and not at all 

 in others. Shells with a rugged or uneven surface have 

 almost always this epidermis. In some it is strong, lami- 

 nated, 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 



