BRACHIOPODA. 



6/1 



genus Camarclla the shell is almost equivalve, the ventral valve having 

 an incurved imperforate beak ; and the dental lamellae converge to form 

 beneath the beak of the ventral valve a small triangular chamber, from 

 which a median septum is continued. A similar median septum is con- 

 tinued in the dorsal valve from the bases of the brachial 

 processes. Lastly, the g"enus Ste?ioscliisma ( = Camaro- 

 phoria, King) possesses a shell like that of Rhyncho- 

 nella, but the teeth in the ventral valve are supported by- 

 converging lamellae, which unite to form a low median 

 septum, while a more pronounced septum is developed 

 in the interior of the dorsal valve. The species -"of 

 this g'enus are abundant in the Devonian rocks of North 

 America, while other species occur in the Carboniferous 

 and Permian rocks. 



Fig. 536.— Stcn- 

 oschisma (Cam- 

 arophoria) globu- 

 lina. Permian. 

 (After King.) 



All the preceding forms are naturally associated 

 with one another by their structural characters ; but 

 there is another great group of Brachiopods usually 

 placed in the Rhynchofiellidce, and agreeing with 

 this family in many points, of which the genus Pentamerus is the 

 type, and which presents certain distinctive features of its own. 

 In Pentamerus (fig. 537) the shell is ovate, the valves articulated by 

 teeth and sockets, and the surface generally ribbed or striated, but 

 sometimes smooth. The beaks are incurved, that of the ventral 

 valve concealing a triangular fissure. Inside the ventral valve " two 

 contiguous vertical septa coalesce into one median plate, extending 



X, 



Fig. 537-— Pentamerus Knightii. The right-hand figure shows the internal septa and 

 dental plates of the shell. Silurian. 



from the beak to a greater or less distance ; and then diverge and 

 form the dental plates, enclosing a triangular chamber of much 

 smaller dimensions than the lateral ones " (Davidson). The small 

 central chamber must have been occupied by the digestive organs, 

 and the spiral arms must have filled the great lateral spaces. In 

 the interior of the smaller or dorsal valve are two longitudinal septa, 

 which often form a chamber corresponding with and apposed to the 

 median chamber in the ventral valve. The Pentameri are confined 

 to the Silurian and Devonian deposits, particular species being often 



