VI 1 1 I'LACODONTIA 3 I I 



remarkable feature is the very elunguti'd siiuaiuusu-jugal arch, which 

 arises inoreover from the dorsal end of tlie long squamoso-quadrate 

 pedicle. The two wide and long temporal fossae are dorsally 

 divided by narrow parietal crests. There is a distinct interparietal 

 bone, and the usual interparietal foramen. The clioanae are 

 united and lie within the palatines, which themselves are united; 

 the large lateral palatal foramina are otherwise enclosed l)y 

 the pterygoids,' quadrates, and laterally by the squamoso-jugal 

 arch. 



Order IV. PLACODONTIA. 



The.se are the latest and last members of the 'J'Jieromorpha, 

 unfortunately known from skulls only, from the Muschelkalk or 

 Middle Trias of Germany and Eussia. The skull of Placodus 

 gigas is about one foot long, rather high and triangular owing 

 to the lateral expansion of the temporal arches, which diverge 

 posteriorly. The squamoso-jugal arch is very broad, and most 

 of the posterior border of the orbit is formed by the large 

 postorbital bone. The maxillary bone seems to extend back to 

 beyond the level of the orbits. The choanae lie behind the pre- 

 maxillaries. The palatines and pterygoids are fused in the 

 middle line, forming a broad bony palate, which, owing to the 

 broad, posteriorly extended wings of the pterygoids, much re- 

 sembles that of the crocodiles. The teeth are very remarkable. 

 There are two or three stout, conical, or chisel-like teeth in each 

 premaxillary bone, and three to five Ijroad and flat maxillary 

 teeth : three pairs of huge, broad, and quite flat teeth are crowded 

 togetlier and fill up the whole vomerine and palatine portion of 

 the palate. These crushing teeth indicate that Placodus proljably 

 lived upon hard-shelled molluscs, and this would be in conformity 

 with its occurrence in the Muschelkalk, which is a strictly marine 

 deposit and full of shells. Another closely allied genus is Cya- 

 modus, one species of which is known from Eussia. The teeth 

 are fewer in number and not so large as those of Placodus. 



