SHELL. 



563 



would seem to be peculiarly abundant, being 

 apparently of the same kind as that of which 

 the byssus of these animals is composed. 



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 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 examina- 

 tion of Jig. 417. will clearly show the mode in 

 which the operation is effected. This 



figure 



Fig. 417. 



ciion of shell of Unio Occident, near the Up, showing the arrangement of the outet or prismatic, 

 internal or nacreous layers: a a', bV, c</, successive Hues of growth; d, margin of the valve 



Vertical section 

 and the 

 Magnified 8 diameters. 



represents a section of one of the valves of 

 Unio occiclens, taken perpendicularly to its 

 surface, and passing from the margin (at the 

 right hand of the figure) towards the umbo 

 (which would be at some distance beyond the 

 left). 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 cells, and passing 

 through the nacreous lining, which is seen to 

 be made up of numerous laminae, separated 

 by the lines aa\ bb', cc', &c. These lines 

 evidently indicate the successive formations 

 of this layer; and it may be easily shown, by 

 tracing them towards the umbo 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. The number of such laminae, 

 therefore, in the oldest part of the shell, in- 

 dicates 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 be- 

 yond 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 for- 

 mations of prismatic structure, each being 

 applied to the inner surface of the preceding, 

 instead of to its free margin. The same ar- 

 rangement may be well seen in the Oyster; 

 with this difference, that the successive layers 

 have but a comparatively slight adhesion to 

 each other. 



The shells of Tcrcbratulce, and of several 



other genera of Brachiopoda, or Palliobr.an- 

 chiate Bivalves, are distinguished by peculiari- 

 ties of structure, which serve to distinguish 

 them from all others. When thin sections 

 of them are microscopically examined, they 

 present a very peculiar texture, (Jig. 418. .) 

 Fin. 418. 



Portion of the shell of Tereliratida australis, showing 

 the orifices of the perforations, and the peculiar 

 structure of the shell : at a the shell is traversed 

 by the section ; at b is shown its internal surface. 



which might be referred either to long flattened 

 cells, or to plications in the shell-membrane ; 

 on the other hand, the natural internal surface 

 of these shells always exhibits an imbricated 

 aspect of great regularity (/;). If the section 

 pass very obliquely towards this surface, it 

 becomes evident that these imbrications are 

 formed by the outcrop of the long flattened 

 cells or folds, which were seen when the plane 

 of the section has passed in the direction of 



o o 2 



