234 



CLASSIFICATION OF VERTEBRATES. 



Fig. 236. Diagrams of cyclospondylous (^) 

 and asterospondylous (^) vertebras. Calcifications 

 of cartilage black. 



line between calcified cartilage and bone. Membrane bones are 

 absent in all recent forms, but in some fossils the dermal scales 



united to form an ex- 

 tensive armor. In 

 some the vertebral 

 centra are entirely of 

 cartilage. When lime 

 is deposited in them 

 it may take two 

 shapes, either laid 

 down in concentric 

 areas (cyclospondy- 

 lous type), or in a ra- 

 dial manner (astero- 

 spondylous). In the diplospondyli and in the tail of some skates 

 an embolomerous condition occurs. In all recent forms the neu- 

 ral arch is converted into a closed canal by the insertion of 

 intercalary pieces betweeen the neural processes and spine. 



The cranium is a solid box without sutures. In its roof 

 there may be one or two gaps (f ontanelles) closed by membrane. 

 The pterygoquadrate is never firmly united to the cranium, but 

 either articulates directly with it (amphistylic, Fig. 237), or is 

 supported by ligaments 

 and by the interven- 

 tion of the hyomandib- 

 ular between the hinder 

 end of the pterygoquad- 

 rate and the otic region 

 of the cranium (hyo- 

 stylic. Fig. 162), thus 

 forming a suspensor 

 for the jaws. The 

 pterygoquadrate forms 

 the upper jaw, but is re- 

 enforced in many spe- 

 cies by labial cartilages. In some extinct elasmobranchs girdles 

 are apparently absent, but in all recent forms they are well 

 developed. The pectoral girdle consists of a simple U-shaped 



Fig. 237. 



Skull of Heptanchus, after Giinther ; 

 amphistylic. 



