162 



BRACHIOPOD A 



into a beak or umbo (fig. 70). The ventral umbo is more 

 prominent than the dorsal, and has generally, either at 

 its apex or just beneath it, an opening. With a very few 

 exceptions the shell of the brachiopod is equilateral, that 

 is to say, a line drawn from the umbo to the opposite 

 margin divides it into two equal and similar parts. This 

 character, combined with the inequality in the size of the 

 valves and the perforation at the umbo, renders it easy to 

 distinguish the shell of a brachiopod from that of a lamel- 

 libranch. In many forms the two valves are joined 

 together by means of a hinge, these constitute the group 

 Articulata ; in others they are held together by the muscles 



Fig. 70. Terebratula semiglobosa, Upper Chalk. A, dorsal view. B, 

 lateral view, a, posterior ; b, anterior ; a — b, length ; c — d, breadth ; 

 e — -/, thickness ; g — h, hinge-line. x §. 



and the mantle only, these form the Inartioulata. The 

 hinge consists of two short curved processes or teeth given 

 off from the ventral valve near the umbo, which fit into 

 corresponding sockets in the dorsal valve. In some genera 

 (e.g. Orthis) the teeth are supported by plates (the dental 

 plates) which are fixed to the inside of the ventral valve. 

 The part of the margin of the valves where the teeth occur 

 and on which the two valves move in the opening and 

 closing of the shell, is termed the hinge-line (fig. 70, g — h). 

 In some genera (Terebratula) this is short and curved, in 



