122 



VERTEBRATE SKELETON 



and relations. The skulls are stegocrotaphic, having the temporal 

 region without fossae. A second feature is the large number of 

 bones and their constancy, points which make these animals impor- 

 tant for a study of the Tetrapod skull (p. ii8). Some of the bones 

 are often sculptured or grooved for lateral hne canals, the grooves 

 in some cases forming a well-defined figure (lyra) between orbits 

 and nares. Viewed from above the skull is triangular, sometimes 

 elongate, sometimes broader than long. The nares, far forwards, are 

 widely separated; the orbits often have a circle of sclerotic bones. 

 The postero-lateral angles of the cranium are frequently prolonged 



Fig. 128. — Cranium of Diplocaulus (Douthitt, 

 '17). do, dermoccipital; /, frontal; m, maxilla; p, 

 parietal; pf, postf rental; po, postorbital; pr, pre- 

 frontal; sq, squamosal; t, tabulare; 2, zygomatic. 



Fig. 129. — Floor of skull of 

 Eryops (Broili). eo, exoccipital; 

 mx, maxilla; pi, palatine; pm, 

 premaxilla; ps, parasphenoid; pt, 

 pterygoid. 



backwards, leaving a groove (sometimes closed to a fenestra) on the 

 medial side of each, interpreted as connected with the auditory 

 apparatus. 



The general relations of the bones of the dorsal surface are shown 

 in figures 126, 128. On either side of the middle line at the hinder 

 end of the cranium is a bone, usually called a supraoccipital, some- 

 times an interparietal, but it is probably a dermoccipital. Lateral 

 to this and reaching to the auditory cleft is a so-called epiotic, but 

 probably a tabulare. There is always a parietal foramen between 

 the parietals; the epiphysial organ which it contained probably was 

 functional. In Eryops (fig. 126) an interfrontal occurs between fron- 

 tal and nasals, but this is rare elsewhere. The circumorbital ring is 

 represented by pre- and postfrontals, postorbital, zygomatic and 

 lacrimal. Quadrate and pterygoid are sometimes visible from above. 



