EXPLANATION OF THE PARTS OF SHELLS. 19 



tally, and are quite open at one end, as in the Patella testudinaria, 

 the Crepidula, and Calyptraea. 



Umbilicus is in general a circular perforation in the base of 

 the lower whorl, or body, of many univalve shells. This is com- 

 mon to most of the Trochi, in some species of which it penetrates 

 from the base to the apex : widest at the base, and gradually 

 tapering to the top. Flate III. fig. 1. u. 



Sub-umbilicated shells are those which have the umbilicus 

 covered in a greater or less degree by a thin process ; which, in 

 some, almost entirely closes the opening or mouth. This charac- 

 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 

 communication between the chambers of the Nautili, and pene- 

 trates 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, 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 spe- 

 cies they have more the form of sutures than ribs ; this is owing 

 to the margin of the outer lip being but slightly developed. 



Jtibs 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 protuberances, are fine 

 white lamina;, 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 1 3. a. 



Epidermis is a skin, or cuticle, covering the exterior surface 

 of shells, destined by nature to protect their surface from being 

 injured. It is membranaceous, somewhat similar to the perios- 

 teum which covers the bones of animals. This substance is the 

 production 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 



