114 BRACHIOPODA. 



inner and middle layers are traversed by canals (fig. 33, 

 e, d, 34) running at right angles to the surface of the 

 shell, and contain prolongations of the mantle. The shell 

 is secreted by the mantle, its outermost border producing 

 the epidermal layer, a zone just within this producing the 

 lamellated layer, and the remainder producing the prismatic 

 layer which gradually encroaches on the preceding. So 

 that the last layer is the only one which can subsequently 

 increase in thickness. In many forms the lamellated 

 layer is absent; and some (e.g. Rhynchonella) possess no 

 canals traversing 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 mainly phos- 

 phate of lime. The canals which traverse these layers 

 are more numerous and more minute than those found in 

 the Articulate forms. The Brachiopods are divided into 

 two orders, (1) Inarticulata, and (2) Articulata. 



ORDER INARTICULATA. 



The valves are not provided with teeth but are held 

 together by muscles only. The intestine is long and ends 

 in an anus. There is no brachial skeleton. 



Lingula. Shell thin, nearly equivalve, compressed, 

 elongate-ovate or quadrilateral, tapering towards the um- 

 bones, slightly gaping at the extremities. Dorsal valve 

 a little shorter than the ventral. Hinge-line slightly 

 thickened. Twelve muscular impressions in each valve, 

 but only feebly marked. Surface of shell smooth or 

 concentrically or radiately striated. Fixation by means of 

 a long peduncle passing between the umbones. Shell 



