SHELLS OF LAMELLIBRANCHS 925 



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 tig. 699 will 

 clearly show the mode in which the operation is effected. This figure 

 represents a section of one of the valves of Unio occidens, taken per- 

 pendicularly 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', 

 Art-. 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 immediate contact with that which preceded it. 



FIG. 699. 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. 



The number of such laminae, therefore, in the oldest part of the shell 

 indicates the number of enlargements 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 applied 

 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- 

 ence, that the successive layers have but a comparatively slight 

 adhesion to each other. 1 



The shells of Terebratulw and of most other Brachiopods are 

 distinguished by peculiarities of structure which differentiate them 

 from those of the Mollusca. When thin sections of them are 

 microscopically examined, they exhibit the appearance of long flat- 

 tened prisms (fig. 700, A, 6), which are arranged with such obliquity 



1 The most important recent work on the shells of Lamellibranchs is that of 

 the lately deceased F. Bernard ; see Bull. Soc. Geol. France, vols. xxiii. and xxiv. 



