514 



THE THEORY OF EVOLUTION 



Fig. 275. — The course of evolution in the brachiopod genus Stropheodonta 

 as shown by rich fossil deposits in Eastern ^Missouri. 



The species Stropheodonta demissa appears throughout the scries. In the Mineola 

 limestone it is connected by a complete series of intermediate stages with S. mineolaensis, 

 a species that thus arose from S. demissa but became extinct. In the Snyder Creek Shale 

 six other species seem to have likewise arisen from S. demissa, as shown by the intermediate 

 stages, only to become extinct while S. demissa continued. The record illustrates a period 

 at which a species for some unknown reason produced several other species and also that 

 the primitive types are sometimes the ones to survive. It shows on a smaller scale the 

 same thing that occurred among cephalopods in which the primitive nautiloid type has 

 survived while the more specialized ammonites have perished. (After original studies by 

 J. S. Williams.) 



I 



