jaws and palatines, in Placodus as few as twenty all told, in Placochelys still fewer. In consequence, the palatines are very large, meeting 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 prevomers.
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 Dinocephalia, 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 bounding 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 supratemporals. The postorbitals are long bones forming nearly the whole posterior
