176 THE MICROSCOPE AND ITS REVELATIONS. 



shells, however, there is not the least approach to the nacreous aspect; 

 nor is there anything that can be described as definite structure; 1 and 

 the residuum left after its decalcification is usually a structureless 'base- 

 ment-membrane. ' 



569. 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 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 specify whether loth 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 ex- 

 amination of Fig. 391 will clearly show the mode in which the operation 

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



I'll * ) /> T * fl J 1 



view the two substances of which the shell is composed; traversing the 



FIG. 391. 



Vertical section of the lip of one of the valves of the shell of Unto: a, 6, c, successive forma- 

 tions of the outer prismatic layer; a', b', c', the same of the inner nacreous layer. 



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', I V , c c', etc. 

 These lines evidently 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 of 

 the shell its whole interior is lined by a new nacreous lamina in imme- 

 diate contact with that which preceded it. The number of such laminae, 

 therefore, in the oldest part of the shell, indicates the number of enlarge- 

 ments which it has undergone. The outer or prismatic layer of the 

 growing shell, on the other hand, is only formed where the new structure 

 projects beyond the margin of the old; and thus we do not find one layer 

 of it overlapping another, except at the lines of junction of two distinct 

 formations. When the shell has attained its full dimensions, however, new 

 laminae of both layers still continue to be added, and thus the lip becomes 

 thickened by successive formations of prismatic structure, each being ap- 

 plied to the inner surface of the preceding, instead of to its free margin. 

 A like arrangement may be well seen in the Oyster; with this differ- 



1 For an explanation of the real nature of what was formerly described by the 

 Author as ' tubular ' Shell-substance, see 816. 



