MOLLUSCOUS ANIMALS GENERALLY. 



175 



FIG. 300. 



566. In all the genera of the Margaritacece, we find the external layer 

 of the shell prismatic, and of considerable thickness; the internal layer 

 being nacreous. But it is only in the shells of a few families of Bivalves, 

 that the combination of organic with mineral components is seen in the 

 same distinct form; and these families are for the most part nearly allied 

 to Pinna. In the Unionid (or e fresh- water mussels '), 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 Ostracece (or oyster tribe) 

 also, the greater part of the thickness of the shell is composed of a ' sub- 

 nacreous' substance ( 568) representing the inner layer of the shells of 

 Margaritaceae, its successively-formed laminae, however, having very lit- 

 tle adhesion to each other; and every one of these laminae is bordered 

 at its free edge by a layer of the prismatic substance, distinguished by 

 its brownish-yellow color. 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 Margaritacece) there is no proper periostracum. Generally 

 speaking, a thin prismatic layer may be detected upon the external sur- 

 face 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, Trigonia, and Solen, and oc- 

 casionally in Anomia and Pecten. 



567. In many other instances, how- 

 ever, nothing like a cellular struc- 

 ture can be distinctly seen in the deli- 

 cate membrane left after decalcifica- 

 tion; and in such cases the animal 

 basis bears but a very small propor- 

 tion to the calcareous substance, and 

 the shell is usually extremely hard. 

 This hardness appears to depend upon 

 the mineral arrangement of the car- 

 bonate of lime; for whilst in the 

 prismatic and ordinary nacreous layer 

 this has the crystalline condition of 

 calcite, it can be shown in the hard 

 shell of PTiolas to have the arrange- 

 ment of arragonite; the difference be- 

 tween 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. 390), in which the carbonate of lime seems to be de- 

 posited in nodules that possess a crystalline structure resembling that of 

 the mineral termed Wavellite. Approaches to this curious arrangement 

 are seen in many other shells. 



568. 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 Mytilacece (or mussel tribe), and with the common 

 Oyster. In the internal layer of by far the greater number of Bivalve 



Section of hinge-tooth, of Mya arenaria. 



