294 COMPARATIVE ZOOLOGY OF CHORDATES 



the fangs extend back parallel to and beneath the roof of the 

 mouth. 



Strep tostylic skulls are also found in the birds, and especially 

 in the parrots. Here the upper beak is freely movable relatively 

 to the brain-case. Some of the Dinosaurs also had strep to- 

 stylic skulls. 



When the quadrate is fixed, and the upper jaw is incapable 

 of separate movement (as in Sphenodon, crocodiles, and 

 mammals, for instance), the skull is described as monimostylic. 



In many Theromorph reptiles, as in mammals, it is common 

 for the post-orbital bar to disappear, and the temporal fossa 



Fig. 146. — Hind view of the skull 

 of Chelone, showing the rela- 

 tions of the post-temporal fossa 

 (through which an arrow is 

 passed into the orbit). 



Fig. 147. — Hind view of the skull 

 of Ornithorhynchus, showing the 

 small post-temporal fossa, indi- 

 cated by an arrow. 



then becomes confluent with the orbit. In Ornithorhynchus, 

 for example, the temporal fossa has extended upwards in the 

 manner described above in other Synapsida, and its upper 

 border is formed by the parietal. It is bounded behind by 

 the squamosal, below by the squamosal and jugal (forming the 

 zygomatic arch), and in front it has no border since it is 

 confluent with the orbit. Behind, the temporal fossa of 

 Ornithorhynchus communicates with a small post-temporal 

 fossa between the squamosal and the auditory capsule (periotic 

 bone) and which opens on the hind face of the skull. This is 

 the last appearance of the post-temporal fossa, for in higher 

 mammals it is obliterated, as, for instance, in the dog. 



