Beptilian Lower Jaws. 581 



by the others ; the aiiicuh\r face points backwards and 

 slightly upwards, and the downwardly directed postarticular 

 process is a scarcely visible knob. 



The prearticular is a long, thin, straight bone covering the 

 iiiner side of the articular, from which it is separated by a 

 scarcely visible suture. It lies inside the angular, and its 

 forward end is covered by the coronoid. 



It is a remarkable fact that in Cynodonts increasing size of 

 the dentary, and of the masticatory muscles which in Cyno- 

 gnathus n)ust be inserted on it, is correlated with a reduction 

 and weakening of the back part of the jaw, which alone 

 articulates with the skull ; not only are the actual bones 

 small, but their attachment to the dentary is weak, they 

 merely rest in a groove in that bone and are often displaced 

 in the fossil skulls. Examination of any higher Cynodont 

 skull with the mandible in place will show that the very 

 powerful masseter had its attachment on the dentary, and can 

 scarcely have had any connection with the angular, and the 

 temporal muscles were obviously inserted in mammalian 

 fashion on the coronoid process of the dentary. The action 

 of these powerful muscles must have set up such great stresses 

 in the hinder part of the jaw that it is doubtful if it could 

 have supported them. Directly correlated with increase in 

 size of tlie dentary and reduction in size of the angular &c. is 

 an increase of tlie descending process of the pter^-goid. It is 

 noteworthy that this takes place not only between the higher 

 and lower Cynodonts, but also in the Tlierocephaliau " Lyco- 

 saurus^^ curvitnola, which pretty certainly comes from the 

 Cisticephalus-zone of Upper Permian age. These processes 

 give rise to the anterior part of the pterygoidal muscles, and 

 their increase in size no doubt implies a corresponding increase 

 in the muscles, which in reptiles are normally attached to the 

 angular. As it is impossible that a muscle should increase 

 whilst its point of attachment is degenerating, it apjjears 

 probable that the pterygoidal muscles were mainly inserted 

 onto the postero-inferior angle of the dentary, which is 

 thickened. Thus inserted these muscles, whilst tending to 

 close the mouth, will produce stresses in the hinder part of 

 the jaw in the opposite direction to those induced by the 

 masseter and temporal muscles, in this way permitting the 

 reduction of the hinder part of the jaw which we actually see. 

 The fact that in higher Cynodonts all the masticatory muscles 

 have their attachments on the dentary renders the freeing of 

 the articular and quadrate demanded by the quadrate = incus 



