PERFORATED SHELLS OF TEREBRATULIDiE. 



601 



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, 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 lamina? 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 adhesion to each other. 



465. The shells of Terebratulce, and of several other Genera of 

 Brachiopoda, are distinguished by peculiarites of structure which 

 serve to distinguish them from all others. "When thin sections of 

 them are Microscopically examined, they exhibit the appearance 

 of long flattened Prisms (Fig 312, a, b), which are arranged with 



Fig. 312. 



a, Internal surface (a), and oblique section (b), of Shell of Tere- 

 bralula (Waldheimia) australis : b, external surface of the same. 



such obliquity that their rounded extremities crop-out upon the 

 inner surface of the Shell in an imbricated (tile-like) manner (a). 

 All true Terebratulidce, both Recent and Fossil, exhibit another 

 very remarkable peculiarity ; namely, the perforation of the 

 Shell by a large number of Canals, which generally pass nearly 

 perpendicularly from one surface to the other (as is shown in ver- 

 tical sections, Fig. 313), and terminate internally by open orifices 



