BRACHIOPODA. 



209 



In the genus Orthis the valves are articulated by teeth and 

 sockets, and usually are more or less transversely oblong (fig. 

 153). There is a straight hinge-line, generally 

 shorter than the width of the shell. Each valve 

 has a hinge-area, notched in its centre by a 

 triangular fissure through which the fibres of 

 the peduncle were transmitted. The shell is 

 often more or less flattened or depressed, and 

 the surface may be smooth, but is more com- 

 monly ornamented with striae, or furnished with 

 well-marked longitudinal ribs. The species of 

 the genus Orthis abound in the Silurian, Devonian, and Car- 

 boniferous periods, especially in the first of these ; but the 

 genus is not known to have survived the Carboniferous period. 



In the genus Orthisina (fig. 156) the shell nearly resembles 

 that of Orthis; but there is a double hinge-area, largest in 

 the ventral valve, the central fissures of 

 which are always covered by a convex delti- 

 dium ; whereas in the latter genus they are 

 open. In some species (as in O. Verneuilt) 

 the deltidium is perforated by a foramen 

 under the beak of the ventral valve. The 

 typical species of Orthisina are Silurian; 

 but the genus is stated by Mr Davidson to 

 range through the Devonian and Carbon- 

 iferous into the Permian. 



In Strophomena (fig. 157) the shell is 

 depressed, generally semicircular, the hinge-line as long as 

 the width of the shell, or longer. The surface may be smooth, 

 but is most commonly striated or ribbed. There is a double 



157. Strophomena antiguata, Silurian. 



hinge-area, which is largest in the ventral valve. Each hinge- 



area has a median notch, which, in the ventral valve, is par- 



tially covered by a deltidium. The ventral valve may be 



o 



