6o THE OSTEOLOGY OF THE REPTILES 



jaws and palatines, in Placodus as few as twenty all told, in Placo- 

 chelys still fewer. In consequence, the palatines are very large, meet- 

 ing each other throughout as do the pterygoids in the median line. 

 The ectopterygoids are very small and the pterygoids are restricted 

 to the posterior part of the palate, widely separated from the pre- 

 vomers. 



The massive cranium has a large temporal opening bounded above 

 by the parietal, below by the united postorbitals and squamosal, 

 with the postfrontal entering into the anterior border. Except for the 

 postfrontals, the structure here, it is seen, is like that of the Dino- 

 cephalia, and possibly has arisen in the same way. The stout lateral 

 bar below the opening is identified by Jaekel in Plachochelys as 

 composed of the squamosal and quadratojugal, by Huene as the 

 supratemporal and squamosal; both views are probably incorrect, 

 since Broili finds only the squamosal, which is in Placodus the more 

 probable. So, also, Huene believes there is an interparietal, which 

 Broili cannot find. 



The nasal only of the roof bones is unpaired in Placodus; possibly 

 the prevomers are also single. There is a large epipterygoid. No 

 tabulars have been found. The premaxillae in Placodus are large, 

 each with three incisor-like teeth. The largest skulls of Placodus are 

 about ten inches long. 



The Skull of the Ichthyosauria 



(Figs. 50, 51) 



The skull of the ichthyosaurs, while retaining not a few primitive 

 characters, has been highly and peculiarly modified in many ways. 

 The greatly elongated premaxilla, unlike those of other aquatic 

 reptiles, is broadly separated above by the very large nasal, and 

 bears numerous teeth; the maxillae are short. All bones are paired. 

 The frontals are small. The very large orbits have the usual bound- 

 ing bones, prefrontal, postfrontal, postorbital, jugal, and lacrimal, 

 but their relations are somewhat changed. The prefrontals are long, 

 the postfrontals are extraordinarily large, articulating in front not 

 only with the whole extent of the frontals but also with the nasals 

 and prefrontals, posteriorly with the so-called supratcmporals. The 

 postorbitals are long bones forming nearly the whole posterior 



