CELLULAR SHELL-STBUCTXJRB 01? BIVALVES. 509 



the Ostracece (or Oyster tribe), the greater part of the shell is 

 composed of a sub-nacreous substance, the successively formed 

 laminae of vvrhich have very little adhesion to each other ; but 

 every one of these laminse is bordered at its free edge by a layer 

 of the prismatic cellular substance, distinguished by its brownish- 

 yellow color. In these and some other cases^ a distinct organic 

 structure is left after the decalcification of the prismatic layer by 

 dilute acid ; and this is most tenacious and substantial, where (as 

 in the Margaritacece) there is no proper periostracum ; as if the 

 horny matter which would have otherwise gone to form this in- 

 vestment, had been diffused as an intercellular substance between 

 the proper cell-walls. Generally speaking, a cellular layer may 

 be detected upon the external surface of bivalve shells, where 

 this has been protected by a periostracum, or has been prevented 

 in any other manner from undergoing abrasion ; thus it is found 

 pretty generally in Ohama, Trigonia, and Solen, and occasionally 

 in Anomia and Pecten. 



338. In many other instances, however, although a cellular 

 arrangement is apparent in sections of the shell, no corresponding 

 structure can be distinctly seen in the delicate membrane left 

 after decalcification. In all such cases, the animal basis bears 

 but a very small proportion to the calcareous deposit, and the 

 shell is usually extremely hard. But there are numerous other 

 cases, in which no distinct traces of cellular structure can be de- 

 tected in the fully-formed shell ; and in which it would seem as 

 if the consolidation of the animal basis by calcareous deposit had 

 taken place whilst the former was as yet in the condition of a 

 uniform gelatinous sarcode, before the commencement of any 

 difterentiation into cell-wall and cell-contents. A very curious 

 appearance is presented by a section of the large hinge-tooth of 

 Mya arenm-ia (Fig. 255), -in which the carbonate of lime seems to 

 be deposited in nodules, not dis- 

 tinctly separated from each other by ^i". sss. 



intervening membrane, and possess- 



ing a crystalline structure resem- 

 bling that of the mineral termed > \ (' 'y 

 Wavellite. Approaches to this curi- ' ' 'l l*-. '' ^ 

 ous arrangement are seen in many ' ■ -"^^ -*^ 

 other shells. 



339. The internal layer of Bivalve 

 shells rarely presents any distinct 

 structure, when examined in a thin 

 section ; and the residuum left after 

 decalcification is usually a structure- 

 less " basement-membrane." This 



form of shell-substance may conse- '^.^^^^^s^i^^'miJ^^ 

 quently be distinguished as mem- section of hinge-tooth of .wyaaima™. 

 branous. In the Margaritacece and 



many other families, this internal layer has a nacreous or iride^ 





