STRUCTURE OF THE VERTEBRATES 133 



The loss of dermal bones is more easily studied with pictures 

 than description. The fish has been largely omitted on account of 

 the complexity of the skull, the teleost particularly having little 

 relation to other forms. The primitive fish had a more typical 

 skull, which was complicated, however, by the bony operculum 

 covering the gills. This gill cover is found in all the fish above 

 the elasmobranchs, and one or two of the opercular bones were 

 carried over to the earliest amphibia. 



Premaxilln 



Nasal 



Maxilla 



Lacrimal 



Frontal 

 Prefrontal 

 Postfrontal 



Orbit 

 Jugal 



Parietal 

 Squamosal 

 uad. -ju 

 Quadrat 



Occipital 



A. Capitosaurus 

 (Stegocephalia) 



B. Seymouria 

 (Extinct Reptile) 



Fig, 70. Primitive Skulls, Dorsal Views. (A) Capitosaurus (Order 



Stegocephalia) a primitive extinct amphibian. (B) Seymouria, a primitive, 



extinct, anapsid reptile. 



The recent amphibia have a very specialized skull structure. 

 The typical primitive form is illustrated by the Stegocephalia 

 (page 50). The earliest amphibian (Archegosaurus) was ex- 

 tremely fish-like, resembling the fossil Crossopterygian in struc- 

 ture. The later members of this primitive order diverged in two 

 directions, (1) toward the unarmored amphibia and (2) toward 

 the reptilian form. Until recent research definitely placed the 

 latter as amphibia thej- were classified as Pro-reptilia. 



The reptiles continued the loss of bones, the elements of the 

 skull becoming reduced from 76 to 26 in the mammal. The reduc- 

 tion in number was gradual, only two pairs of bones being found 



