STRUCTURE OF SHELL OF BIVALVES. 597 



calcareous concretions form themselves in an organic matrix, have 

 been carefully studied by Mr. Rainey; whose researches (of which 

 some account will be given hereafter, § 599) are worthy of more 

 attention than they have received.* 



460. It is only in the Shells of a few families of Bivalves, that 

 the combination of Organic with Mineral components is seen in 

 this very distinct form ; and these families are for the most part 

 nearly allied to Pinna. In all the Genera of the Mavgaritacece, 

 we find the external layer of the shell Prismatic, and of con- 

 siderable thickness ; the internal layer being Nacreous. In the 

 Unionidas (fresh-water Mussels), on the contrary, nearly the whole 

 thickness of the shell is made-up of the internal or Nacreous layer; 

 but a uniform stratum of Prismatic substance is always found 

 between the Nacre and the Periostracum, really constituting the 

 inner layer of the latter, the outer being simply horny. In the 

 Oslraccce (or Oyster tribe), the greater part of the shell is com- 

 posed of a Sub-Nacreous substance, the successively- formed laminse 

 of which have very little adhesion to each other; but every one of 

 these laminag is bordered at its free edge by a layer of the Pris- 

 matic substance, distinguished by its brownish-yellow colour. In 

 these and some other cases, a distinct membranous residuum is left 

 after the decalcification of the prismatic layer by dilute acid ; and 

 this is most tenacious and substantial, where (as in the Marga- 

 rtiacece) there is no proper periostracum. Generally speaking, a 

 thin Prismatic 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 Chama, Trlgonia, 

 &ii<\. Solen, and occasionally in Anomia and Pecten. 



461. In many other instances, however, nothing like a Cellular 

 structure can be distinctly seen in the delicate Membrane left after 

 Decalcification ; and in such cases the Animal basis bears but a 

 very small proportion to the Calcareous substance, and the Shell is 

 usually extremely hard. This hardness appears to depend upon the 

 Mineral arrangement of the Carbonate of Lime ; for whilst, in the 

 prismatic and ordinary nacreous layer this has the crystalline con- 

 dition of Calcite, it can be shown in the hard shell of Pholas to 

 have the ai-rangenient of Arragonite ; the difference between the 

 two being made evident by Polarized light. A very curious 

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

 My a arenaria (Fig. 309), in which the Carbonate of Lime seems 

 to be deposited in nodules that possess a crystalline structure re- 

 sembling that of the Mineral termed Wavetlite. Approaches to 

 this curious arrangement are seen in many other Shells. 



462. The internal layer of Bivalve Shells rarely presents any 



* See his Treatise " On the Mode of Formation of the Shells of Animals, 

 of Bone, and of several other structures, by a Process of Molecular Coa- 

 lescence, demonstrable in certain artificially-formed Products," 18J8. 



