BRACHIOPODA 



169 



Fig. 74. Horizontal section 

 through the prismatic layer of 

 Terebratula maxillata, from the 

 Great Oolite, showing prisms 

 and canals. Magnified. 



being encased in a membrane, which of course has dis- 

 appeared in the fossil examples. The middle layer (c) is 

 lamellated and also calcareous. The outer (b) consists 

 of chitinous material. The 

 inner and middle layers are 

 traversed by canals (figs. 73, 

 e, d, 74) running at right 

 angles to the surface of the 

 shell, and containing pro- 

 longations of the mantle ; in 

 fossil specimens, in which 

 the chitinous layer is not 

 preserved, the openings of 

 these canals can be seen on the surface of the shell, giving 

 it a punctate appearance. The shell is secreted by the 

 mantle, its outermost border producing the chitinous 

 layer, a zone just within this forming the lamellated layer, 

 and the remainder giving rise to the prismatic layer which 

 gradually encroaches on the preceding; hence the last 

 layer is the only one which can subsequently increase in 

 thickness. In many forms the lamellated layer is absent, 

 and in some (e.g. Rhynchonella) there are no canals tra- 

 versing the calcareous layers. 



The shell of the Inarticulata has a different structure. 

 In Lingula it consists of alternating calcareous and chiti- 

 nous layers, the calcareous material being largely phos- 

 phate of lime ; the canals which traverse these layers 

 are more numerous and much smaller than those found in 

 the articulate forms. In Crania the shell is calcareous 

 and the canals branch near the surface. 



The development of the shell in the Brachiopoda has 

 been studied by Beecher. In the earliest or embryonic 

 stage the shell is similar in character in all the genera 



