SKULL. 



375 



is formed by the basi-occipital only, and the exoccipitals meet 

 above the foramen magnum. The parietals and f rentals are 

 unpaired in the adult. The nostrils are single, placed at the 

 front end of the skull and entirely surrounded by the premaxil- 

 laries. The nasals are long, but do not reach the anterior nares. 

 They usually reach the premaxillaries, but in Gavialis are separ- 

 ated from them by the maxillaries. The vomers are double 

 and by their opposed vertical plates separate the narial passages. 

 The premaxillae, maxillae, and palatines develop palatal plates 

 which unite in the middle line and form a 

 hard palate, and in all living crocodiles and 

 in extinct crocodiles from the Cretaceous 

 onwards, the pterygoids are modified in the 

 same way, so that the internal narial open- 

 ing is placed far back on the base of the skull 

 (Eusuchia of Huxley, Fig. 205). In precre- 

 taceous crocodiles the pterygoids do not de- 

 velop palatal plates and the posterior nares 

 are behind the palatines (Mesosuchia of 

 Huxley. Fig. 204). An ectopterygoid or 

 transpalatine connects the pterygoid with 

 the maxilla and jugal (Fig. 204, T). 



Prefrontals, lacrymals (unperf orated), and 

 postf rentals are present. The orbit is sepa- 

 rated from the temporal fossa by the post- 

 orbital bar formed by processes of the post- 

 frontal and jugal. Both supra- temporal and 

 infra-temporal arcades are present, the 

 former constituted by the postfrontals and 

 squamosals, the latter by the jugal and quad- 

 rato-jugal. The quadrate is large, projects backwards and is 

 overlaid by the squamosal ; it is immoveably fixed to the skull. 



The tympanic cavity is well surrounded by bone. It com- 

 municates with the pharynx by a complicated system of eusta- 

 chian canals, and gives off air-passages into the surrounding 

 bones. Of these, the air-passages of the quadrate communicate 

 by a membranous tube (siphonium) with an air space in the 

 os articulare of the mandible, and a passage running through the 

 bones of the posterior part of the roof of the skull puts the two 

 tympanic cavities in communication. 



FIG. 204. Ventral 

 view of skull of 

 Pdagosaurus tern- 

 poralis Blv., to 

 show the meso- 

 suchal choanae 

 (after Zittel). Bo 

 basioccipital ; Ch 

 choanae ; J jugal; 

 MX maxilla ; PI 

 palatine ; P t 

 pterygoid ; Q u 

 quadrate; T 

 traaspalatine (ec- 

 topterygoid). 



