924 



MOLLUSCA AND BBACHIOPODA 



brownish-yellow colour. In these and some other cast's a distinct 

 membranous residuum is left after the decalcificatioii of the prismatic 

 layer by dilute acid ; and this is most tenacious and substantial 

 where (as in the Margaritacece) there is 110 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 mariner 

 from undergoing abrasion ; thus it is found pretty generally in 

 Chama, Trigonia, and Solen, and occasionally in Anomia and Pecten. 

 In many other instances, however, nothing like a cellular struc- 

 ture can be distinctly seen in the delicate membrane left after decal- 

 cification ; and in such cases the animal basis bears but a very small 

 proportion to the calcareous substance, and the shell is usually ex- 

 tremely hard. This hardness ap- 

 pears to depend upon the mineral 

 arrangement of the carbonate of 

 lime ; for whilst in the prismatic 

 and ordinary nacreous layer this 

 has the crystalline condition of 

 calcife, it can be shown in the hard 

 shell of Pholas to have the arrange- 

 ment of arragonite, the difference 

 between the two being made evi- 

 dent by polarised light. A very 

 curious appearance is presented by 

 a section of the large hinge-tooth 

 of Mi/a arenaria (fig. 698), in 

 which the carbonate of lime seems 

 to be deposited in nodules that 

 possess a crystalline structure re- 

 sembling that of the mineral 

 termed wavellite. Approaches to 

 this curious arrangement are seen in many other shells. 



There are several bivalve shells which almost entirely consist of 

 what may be termed a sub-nacreous substance, their polished 

 surfaces being marked by lines, but these lines being destitute of 

 that regularity of arrangement which is necessary to produce the 

 iridescent lustre. This is the case, for example, with most of the 

 Pectinidce (or scallop tribe), also with some of the Mytiluwii' (or 

 mussel tribe), and with the common Oyster. In the internal layer 

 of by far the greater number of bivalve shells, however, there is not 

 the least approach to the nacreous aspect ; nor is there anything 

 that can be described as definite structure ; and the residuum 

 left after its decalcificatiori is usually a structureless ' basement 

 membrane.' 



The ordinary account of the mode of growth of the shells of 

 bivalve Mollusca that they are progressively enlarged by the depo- 

 sition of new laminae, each of which is in contact with the internal 

 surface of the preceding, and extends beyond it does not express 

 the whole truth ; for it takes no account of the fact that most shells 

 are composed of two layers of very different texture, and does not 



PIG. 698. Section of hinge-tooth of 

 Mi/a arenaria. 



