ORDER V 



THEKOMORPHA 



185 



fused with the small quadrate below. A foramen is enclosed by the parietals, 

 which are small, and sometimes much reduced. The frontal and prefrontal 

 form the superior border of the orbit, and the maxilla, lachrymal, and pre- 

 frontal the anterior. The orbits are laterally placed in about the middle of 

 the skull, and in one genus (Ptychognathus) traces of a sclerotic ring have been 

 observed. A narrow, curved bar, the 

 postorbital, separates the orbit from 

 the supratemporal vacuity, and the 

 lower orbital boundary is formed by 

 the maxilla together with a very 

 massive jugal. 



The nasals are steeply inclined 

 toward the front, and form with the 

 unpaired premaxilla a sometimes elon- 

 gated rostrum. Small internasals are 

 occasionally present below the anterior 

 nares, which are placed laterally at the 

 junction of the nasals, lachrymals, and 

 premaxillae. The oral border of the 



Fig. 290. 

 Ptvchognathn.<< dedims, Owen. Karoo Formation; Rhenosterberge, Cape Colony. Skull from lateral {A) 

 superior (B), and posterior (C) aspects, 1/3. A, Orbit; ang, Angular ; bo, Basioccipital ; e, MaxUlary tusk ; a 

 Dentary ; exo, Exoccipital ; fr, Frontal; j«, Jugal; mx, Maxilla; N, External narial opemng- -'- 

 Parietal ; jw/ia;, Premaxilla ; prf, Prefrontal ; pf/, Postfrontal ; qu, Quadrate 

 (after Owen). | 



na, Nasal ; pa, 

 Supraoccipital ; sq, Squamosal 



latter forms a sharp, cutting edge, and was probably sheathed during life of 

 the animal by a corneous layer, as in Chelonians and Aves. A pair of 

 powerful, decurved, tusk-like teeth, one on each side, is deyeloped by the 

 maxillae, and the deep alveoli in which they are seated are indicated by an 

 external folding of the plate. Even among toothless forms alveoli are present, 

 but are occupied by osseous substance. 



The basioccipital is short and takes part in the occipital condyle. In front 

 of it on the lower side (Fig. 291) is placed a subrectangular basisphenoid which 

 joins the very large pterygoids. The latter meet in the middle line, but leave 

 a small interpterygoidal vacuity behind the narrow vomer. The backwardly 



