BRACHIOPODA. 



121 



Pentamerus (fig. 41). Shell oval, ornamented with 

 ribs, rarely smooth. Ventral valve the most convex. 

 Dorsal valve usually with a fold in front and on the 

 ventral valve a corresponding depression. Umbone of 



A B 



FIG. 41. Pentamerus knighti, Aymestry Limestone. A, longitudinal 

 section ; B, transverse section, ss, dorsal septa ; , ventral septum ; 

 d, dental plates. (From Woodward.) 



ventral valve sharp, strongly incurved, usually touching 

 the dorsal valve and concealing the triangular aperture. 

 No area, nor deltidium. Dental-plates (d) trough-like, 

 converging in the ventral valve to form a median septum 

 (s), in the dorsal valve two contiguous septa (ss). Silurian 

 to Devonian. 



Terebratula (fig. 42). Shell biconvex ; oval, elongate, 

 or rounded ; surface smooth. Often with two folds on 



FIG. 42. Terebratula vitrea, Kecent. Interior of dorsal valve, showing 

 the brachial skeleton. (From Woodward.) 



