924 



MOLLUSCA AND BBACHIOPODA 



brownish-yellow colour. In these and some other cases 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 Jfargaritacete) 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 manner 

 from undergoing abrasion; thus it is found pretty generally in 

 Ghama, Triyonia, and >Solen, and occasionally in Anomia and /Vr/r//. 

 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 



PIG. 098. Section of hinge-tooth of 

 M//H arenaria. 



lime ; for whilst in the / 

 and ordinary inict-miis layer this 

 has the crystalline condition of 

 rti/'rlte, it can be shown in the hard 

 shell of PJiolxs to have tlie arrange 

 meiit 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 Mya 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 iravellite. 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 i.s the case, for example, with most of the 

 /'rrflni/fti- (or scallop tribe), also with some of the Jft/til<t<-<><f (or 

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

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

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

 that can be described as definite structure: and the residuum 

 left after its decalcilication is usually a structurele -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 \\ liirh is in contact with the internal 

 surface of the preceding, and extends beyond it doe> not express 

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

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



