BRACHIOPODA. 109 



ventral valve near the umbone, which fit into correspond- 

 ing sockets in the dorsal valve. The line near the teeth 

 on which the two valves move in the opening and closing 

 of the shell, is termed the hinge-line (fig. 30, g A). In 

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

 others (Spin/era) it is long and straight. The posterior 

 part of the shell is that near the hinge (fig. 30, a), the 



FIG. 30. Terebratula obesa, Upper Chalk. A, dorsal view. B, lateral 

 view, a, posterior; b, anterior; a b, length; cd, breadth; e f, 

 thickness ; g h, hinge-line. Two-thirds natural size. 



anterior is the opposite margin (6). The length of the 

 shell is measured from the posterior to the anterior 

 border (a 6). The breadth is at right angles to this, from 

 one side of the shell to the other (c d). The thickness is 

 measured from one valve to the other perpendicular to the 

 length and breadth e -f. In some genera (e.g. Terebratula) 

 the length is greater than the breadth, in others (e.g. 

 Strophomena) the bread this greater. Between the hinge- 

 line and the umbone, there is in some brachiopods (e.g. 

 Cyrtia, fig. 31) a flat or slightly concave portion of the 

 shell, usually triangular, on which the ornamentation of 

 the rest of the shell is absent, the surface being either 

 smooth or striated ; this is known as the area. It is 



