94 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



The OPHIDIA (snakes) lack parotic process, parietal foramen, temporal arcades 

 and epipterygoid, and have the squamosal excluded from the cranial wall. The 

 attachment of the visceral skeleton to the cranium is loose, the pterygoid being 

 connected to the other parts by a long bar, consisting of squamosal and quadrate 

 behind and by transversum and palatine in front, features related to the great 

 distensibility of the jaws. In the poisonous serpents the poison fangs are either 

 permanently erect, or they fold back when the mouth is closed. In the latter the 

 fangs are supported on the maxillaries, which are moved by a rod formed of quad- 

 rate, pterygoid and ectopterygoid. In the lower jaw distensibility is provided for 

 by the elastic ligament connecting the two halves in front. Some species have 

 remnants of the hyoid apparatus, but occasionally all are lost in the adult. 



FIG. 97. Skull of snake, Tropidonotus, after W. K. Parker. 



When the whole series of CROCODILIA, recent and extinct, is considered the 

 range of variation in the skull is considerable. In all, supra- and infratemporal 

 fossae are present, the quadrate is immovable, there is more or less of a secondary 

 palate, no parietal foramen, and the thecodont teeth are confined to the margins 

 of the jaws. In the complete series the gradual change of position of the choanas 

 can be traced from the oldest in which they are beside the vomers; then in the meso- 

 suchia the palatines meet in the middle line, carrying the choanae back as a single 

 opening behind these bones; while in the recent species the pterygoids have also 

 met, so that the choanae are between them and the basisphenoid. 



Among the recent species the basioccipital is excluded from the foramen mag- 

 num, pre- and orbitosphenoids are imperfectly ossified, the nasals are long and the 



