ADAMS, PHYLOGENY OF THE JAW MUSCLES 155 



either at the top or at the sides or in both regions at once; much as in 

 istricomorph rodents, a slip of the masseter has invaded the region of 

 the infraorbital foramen, so that it now extends through a widely open 

 ircade and finds room for expansion on the side of the face. 



(3) A comparative study of the skull of Tyrannosaurus led to the 

 suspicion that the pre-orbital fenestras of the dinosaurs, phytosaurs, pte- 

 rosaurs, etc., were also functionally connected with the muscles of masti- 

 cation; but it was realized that proof of this view required a wider study 

 of the jaw-muscles of living reptiles. It was afterwards found that Dollo 

 (1884) had suggested that the pre-orbital fenestras of extinct reptiles 

 were filled by the pterygoid muscles. 



(4) The inferred conditions of the jaw musculature of Cynognabhus 

 are in harmony with the view that in the mammal the back part of the 

 reptilian jaw became transformed into the accessory auditory ossicles. 



(5) As a working hypothesis, it is assumed that the transformation 

 of certain elements in the temporal and occipital regions of early Tetra- 

 poda was partly conditioned by the stresses induced upon the skull-roof 

 by the jaw and neck muscles. Comparison with lizards, Sphenodon, etc., 

 clearly indicates that the prolongation of the parietal into the postero- 

 external process, joining the true squamosal, was correlated with the 

 squeezing effect of the capiti-mandibularis and depressor mandibulas 

 muscles. This may also be responsible for the suppression of the supra- 

 temporal and survival of the squamosal in early reptiles. The shifting 

 of the post-parietals (dermo-supraoccipitals) and tabularia from the 

 dorsal to the posterior aspect of the occiput was no doubt influenced also 

 by the forward growth of the neck muscles upon the occiput. 



