178 



BRACHIOPODA 



Pen tamer us (fig. 81). Shell oval, or subtrigonal, biconvex, 

 ornamented with ribs, rarely smooth. Ventral valve the more 

 convex. Dorsal valve sometimes with a fold in front, ventral valve 

 with a corresponding depression. Umbo of ventral valve large, sharp, 

 often strongly incurved, usually touching the dorsal valve and 

 concealing the delthyrium. Usually no area and no pseudo-deltidium ; 



A B 



Fig. 81. Pentamerus (Conchidium) knighti, Aymestry Limestone. A, 

 transverse section. B, longitudinal section, s, septa ; d, dental 

 plates. (From Woodward.) x J. 



hinge-line curved. Dental plates (d) trough-like, converging in the 

 ventral valve to form a large median septum (s) ; in the dorsal valve 

 two septa close together (s-s). Silurian to Devonian. Ex. P. oblo?igus, 

 Llandovery; P. knighti, Aymestry Limestone; P. galeatus, Wenlock 

 Limestone. Conchidium and Oypidida are sub-genera of Pentamerus 

 as defined above, but are regarded by some writers as independent 

 genera. 



Stricklandia. Shell large, oval, ornamented with ribs ; valves 

 nearly equal, sometimes with a fold and a sinus. Umbo of ventral 

 valve not prominent. An area on each valve, the dorsal being 

 small. A short median septum in the ventral valve from which 

 arise two plates forming a small chamber under the umbo. Silurian. 

 Ex. S. lens, Llandovery Beds. 



Camarophoria. External form similar to Rhynchonella. In 

 the ventral valve the dental plates converge to form a short trough 

 supported by a long median septum. In the dorsal valve there is a 

 trough-like plate supported by a septum. Devonian to Permian. 

 Ex. G. schlotheimi, Permian. 



