236 



DR. CARPENTER, ON THE MICROSCOPE 



may be regarded as exclusivel}' his own. Such is his account 

 of the Structure of Shells, from which we give an extract re- 

 lating to the forms assumed by the Brachiopoda. 



" 341. The shells of Terehratuke, and of several other genera of Bra- 

 chiopoda, are distinguished by peculiarities of structure, which serve to 

 distinguish them from all others. When thin sections of them are micro- 

 scopically examined, they exhibit the aj^pearance of long flattened prisms 

 (fig. 259, b), which are arranged with such obliquity, that their rounded 

 extremeties croj) out upon the inner surface of the shell in an imbricated 

 (tile-like) manner («). All true Terehratulidce, both recent and fossil, ex- 

 hibit another very remarkable peculiarity ; namely, the presence of a large 

 number of perforations in the shell, generally passing nearly perpendicu- 

 larly from one surface to the other (as is sliown in vertical sections (fig. 

 261), and terminating internally by open orifices (fig. 259), whilst exter- 



Fig. 259. 



X 



Fig. 260. 



^^SSJf ^-~^^i^ 





Fig. 259. Internal surface (a), and oblique section (I)), of Shell of TereJyi-a- 

 tula (Waldheimia) australis. 

 Fig. 260. External surface of the same. 



261. 



nally they are covered by the periostracum (fig. 260). Their diameter is 

 greatest towards the external 

 surface, where they sometimes 

 expand suddenly, so as to be- 

 come trumpet-shaped ; and it 

 is usually narrowed rather sud- 

 denly, when, as sometimes hap- 

 pens, a new internal layer is 

 formed as a lining to the pre- 

 ceding (fig. 261, A, d d). Hence 

 the diameter of these canals, 

 as shown in different transverse 

 sections of one and the same 

 shell, will vary according to the 

 part of its thickness which the 

 section happens to traverse. 

 The different species of Tere- 

 hratulidce, however, ^^r^sent 

 very striking diversities in the 

 size and closeness of the canals, 

 as shown by sections taken in 

 corresponding parts of their 

 shells ; three examples of this 

 kind are given for the sake of comparison in figs. 262-264 



t 





ycitical sections of Shell of Ttnhiatala (Wald- 

 heimia) australis : — showing at A the canals open- 

 ing by large trumpet-shaped orifices on the outer 

 surface, and contracting at dd into narrow tubes ; 

 and presenting at b a bifurcation of the canals. 



These canal 



