40 THE ELEMENTS 



univalve shells. Plate 3d, fig. 12tli, 



jR, Jx J\ xv. 



Teeth of univalves, according to 

 Mr. Montagu, are not properly den- 

 ticles, or tooth-sbape protuberances, 

 but are fine white laminw^ or ridges, 

 running spirally backwards, in a pa- 

 rallel direction to each other ; those 

 on the exterior lip may, in most in- 

 stances, be traced through the outside 

 of the shell, and are nearly ahke in 

 length. Plate 3d, fig. 13th, a. 



Mpidermis is a skin, or cuticle, with 

 which the exterior surface of many 

 of the univalve and bivalve shells are 

 covered ; and it is destined by nature 

 to protect the surface of their shells 

 from being injured. It is membrana- 

 ceous, or similar to the periosteum, 

 which covers the bones of animals. 

 This skin seems to be formed entirely 

 by the animal, and is uniformly ob- 

 served in some species, and not at all 

 in others ; but those shells with a 



