600 



GROWTH OF SHELLS OF BIVALVES. 



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

 (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 Shells, there is not the least 

 approach to the Nacreous aspect ; the presence of true Nacre being 

 exceptional, save in a small number of Families.* 



464. The ordinary account of the mode of growth of the Shells 

 of Bivalve Mollusca, — that they are progressively enlarged by 

 the deposition of new laminae, each of which is in contact with the 

 interna] 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 specify whether both these layers are thus 

 formed by the entire surface of the Mantle whenever the shell has 

 to be extended, or whether only one is produced. An examination 

 of Fig. 311 will clearly show the mode in which the operation is 



Fig. 311. 



Vertical section of the lip of one of the valves of the shell of 

 Unio:—a, b, c, successive formations of the Outer Prismatic layer; 

 a', b', c', the same of the inner Nacreous layer. 



effected. This figure represents a section of one of the valves of 

 Unio oscidens, taken perpendicularly to its surface, and passing 

 from the margin or lip (at the left hand of the figure) towards the 

 hinge (which would be at some distance beyond the right). This 

 section brings into view the two substances of which the Shell is 

 composed ; traversing the outer or Prismatic layer in the direction 

 of the length of its prisms, and passing through the Nacreous 

 lining in such a manner as to bring into view its numerous 

 laminae, separated by the lines a a', b b', c c', &c. These lines evi- 

 dently indicate the successive formations of this layer ; and it may 

 be easily shown, by tracing them towards the hinge on the one side 

 and towards the margin on the other, that at every enlargement 



* For an explanation of the real nature of what was formerly described 

 by the Author as ' Tubular Shell-substance,' see § 267. 



