CONCHIFERA. 



215 



valves; it is very contractile, and along its free edges is 

 often studded with sensitive tentacles also contractile ; on 

 the inner surface it is covered with a shining epithelium 

 or internal epidermis. The shells of the conchifera consist 

 for the most part of two distinct layers ; the outer one is 

 formed of prismatic cells*, which stand perpendicularly to 

 the surface of the mantle, and are filled with carbonate of 

 lime ; the inner one is without cellular structure, and con- 

 sists of numerous scales or lamina? lying one above the other, 

 among which granules of lime are deposited. Sometimes 

 the outer, sometimes the inner predominates in the sub- 

 stance of the shell. Both retain their distinctive appear- 

 ance in the fossil; the inner, by its laminated nature, 

 and larger portion of animal matter, showing its struc- 

 ture most clearly, and the external one being often changed 

 into spar, or disappearing altogether ; indeed, a shell con- 

 taining a large proportion of animal membrane, often 



* Dr. Carpenter has the following observations on this structure, in 

 a paper read to the Geological Section of the British Association 

 in 1843. Speaking of the membranous basis of bivalves after the 

 calcareous portion is removed by acids, he says : — " This consists of a 

 stratum of prismatic cells adherent together at their sides, and form- 

 ing by their extremities the two surfaces of the membrane, which 

 thus presents the appearance of a honey-comb. This is most charac- 

 teristically seen in Pinna, nearly the whole of which shell is, formed 

 of this prismatic cellular substance, as may be easily shown by 

 examining the thin laminae into which it separates, without any pre- 

 paration, by the aid of even an ordinary microscope. These cells are 

 usually hexagonal in form, but are not very regularly so. Their 

 size varies considerably, both as to diameter and length, even in the 

 same shell. When very long, the layer which they form will possess 

 considerable thickness ; when they are very short, it will be delicately 

 thin." This structure is observable in the following genera : — Pinna, 

 Avicula (Meleagrina) Perna, Malleus, Crenatula, Ostrea, Etheria, 

 Unio, Anodon, and Pandora. In fossil specimens of the above genera 

 the prismatic structure is well preserved. 



