SHELLS OF TEREBRATUL^. 



513 



&c. ■ 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. The number of such laminse, 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 laminse 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 difference, 

 that the successive layers have but a comparatively slight adhe- 

 sion to each other. 



341. The shells of Terehratulce, and of several other genera of 

 BracMopoda, are distinguished by peculiarities of structure, which 

 serve to distinguish them from all others. When thin sections 

 of them are microscopically examined, they exhibit the appear- 

 ance of long flattened prisms (Fig. 259, b), which are arranged 

 with such obliquity, that their rounded extremities crop out upon 

 the inner surface of the shell in an imbricated (tile-like) manner 

 (a). All true TerebratuUdce, both recent and fossil, exhibit another 

 very remarkable peculiarity; namely, the presence of a large 

 number oi perforations in the shell, generally passing nearly per- 

 pendicularly from one surface to the other (as is shown in vertical 

 sections, Fig. 261), and terminating internally by open orifices 



Fig. 259. 



Fig. 260. 



Fig. 269. Internal surface (a), and oblique seclion (6), of Shell of Terebratula ( Waldlieimia) australis. 

 Fig. 260. External surface of same. 



(Fig. 259), whilst externally they are covered by the periostracum 

 (Fig. 260). Their diameter is greatest towards their external 

 surface, where they sometimes expand suddenly, so as to become 



33 



