STRUCTUEE OF SHELLS 923 



careous portion, 110 such repetition of membranous layers is to be 

 found ; on the contrary, if the piece of nacre be the product of one 

 act of shell formation, there is but a single layer of membrane. This 

 layer, however, is found to present a more or less folded or plaited 

 arrangement, and the lineation of the nacreous surface may perhaps 

 be thus accounted for. A similar arrangement is found in pearls, 

 which are rounded concretions projecting from the inner surface of 

 the shell of Meleayrina, and possessing a nacreous structure corre- 

 sponding to that of ; mother-of-pearl.' Such concretions are found in 

 many other shells, especially the fresh-water mussels, Unio and Ano- 

 don ; but these are usually less remarkable for their pearly lustre ; 

 and, when formed at the edge of the valves, they may be partly or 

 even entirely made up of the prismatic substance of the external 

 layer, and may be consequently altogether destitute of the pearly 

 character. 



In all the genera of the Mwgaritacecv we find the external layer 



FIG. 697. Section of nacreous lining of shell of Meleagrlna 

 margaritifera (pearl-oyster). 



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 Unionidce (or '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 Ostreacece (or oyster tribe), also, the greater 

 part of the thickness of the shell is composed of a ' sub-nacreous ' 

 substance, representing the inner layer of the shells of Margaritaceae, 

 its successively formed laminae, however, having very little 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 



