926 



MOLLUSCA AND BEACHIOPODA 



tliat their rounded extremities crop out upon the inner sin-face 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, 



A 



B 



FIG. 700. A, internal surface, a, and oblique section, b, of shell of ]]'tilrUici>nia 

 a a \t rii I is ; B, external surface of the same. 



which generally pass nearly perpendicularly from one sin-face to the 

 other (as is shown in vertical sections, fig. 701), and terminate inter- 

 nally by open orifices (fig. 700, A), whilst externally they are covered 



by the periostracuin (B) . Their 

 diameter is greatest towards 

 the external surface, where 

 they sometimes expand sud- 

 denly, so as to become trum- 

 pet-shaped ; and it is usually 

 narrowed rather suddenly 

 when, as sometimes happens, 

 a new internal layer is formed 

 as a lining to the preceding 

 (fig. 701. A.,dd). Hence the 

 diameter of these canals, as 

 shown in different transverse 

 sections of one and the same 



shell, will vary according to 

 701 Vertical sections of shell of Wuhl- f ., ", . , 



k,La. austraUs, showing at A the canals th '' P ari ^ ^ thu-kness which 

 opening by large trumpet-shaped orifices the section happens to tra- 

 on the outer surface, and contracting at verse. The shells of different 

 il, il into narrow tubes ; and showing; at TJ e c j T> 7 



a bifurcation of the canals. s P ecies (lt perforated Brachio- 



pods, however, present very 



striking diversities in the size and closeness of their canals, as shown 

 by sections taken in corresponding parts; three examples of this 

 kind are given for the sake of comparison in figs. 702-704. These 

 canals are occupied in the living state by tubular prolongations of 

 the mantle, whose interior is filled with a fluid containing minute 

 cells and granules, which, from its corresponding in appearance with 

 the fluid contained in the great sinuses of the mantle, may perhaps 



