94 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
The Opnip1a (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. 
Fic. 97.—Skull of snake, Tropidonotus, after W. K. Parker. 
When the whole series of Crocoprria, recent and extinct, is considered the 
range of variation in the skull is considerable. In all, supra- and infratemporal 
fosse 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 choane 
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 choane back as a single 
opening: behind these bones; while in the recent species the pterygoids have also 
met, so that the choanz 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 
