512 FOSSIL BRACHIOPODA chap, xviij 



genus and Mesozoic Rhynchonellae ; and a whole series of genera 

 exhibit intermediate stages of structure between the Rhyncho- 

 nelloid and Pentameroid groups. The Terebratuloids can be 

 traced back to the primitive type Renssoellaria ; and amongst 

 spire-bearing forms, the protean genus Spirifera can be split up 

 into groups of species which diverge along lines tending to forms 

 no longer congeneric. When we come to deal with specific 

 differences we find frequently such a host of intermediate varie- 

 ties that the separation of many species, as in the case of Mesozoic 

 Terebratulae, is to a large extent arbitrary and artificial. 



