SILURIAN ANIMALS. 



315 



functions of respiration and alimentation. These arms are attached 

 to a curious bony apparatus, sometimes itself spiral in form. Figs. 

 335-337 show the internal structure described above. 



Fig. 343.— Diagram showing the General Relation in Time of Brachiopods to Lamellibranchs. 



In the present seas the Lamellibranchs are extremely abundant, 

 while the Brachiopods are nearly extinct, being represented by very 

 few species. In Silurian times, on the contrary, the very reverse is 

 the case, bivalve shells being represented mostly by Brachiopods. 



Fig. 344. 



Fig. 346. 



Fig. 347. 



Figs. 344-347.— Silurian Brachiopods: 344. Orthis Davidsonii. 345. Orthis porcata. 346. Spiri- 

 fer Cumberlandise : a, ventral valve; b, dorsal valve; c, suture. 347. Pentamerus Knightii. 



Taking the number of bivalve species throughout geological times as 

 constant, then the general relation of these two sub-classes to each in 

 time may be roughly represented by the following diagram, in which 

 the lower triangle represents Brachiopods, the upper Lamellibranchs, 

 and the common diagonal the line of decrease of one and increase of 

 the other. 



