ADAMS, PHYLOGENY OF THE JAW MUSCLES 137 



give much aid in lowering the mandible. Watson (1912, pp. 581-582) 



As it is impossible that a muscle should increase while its point of attach- 

 ment is degenerating, it appears probable that the pterygoid muscles were 

 mainly inserted onto the postero-inferior angle of the dentary, which is thick- 

 ened. Thus inserted, these muscles, while tending to close the mouth, would 

 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 theory of the mammalian ossicula auditus much more 

 understandable. 



It would seem that Watson's hypothesis that all of the muscles were on 

 the dentary might be questioned, for the cast of the skull of Cynognathus 

 shows plainly the insertion of the depressor mandibular on the posterior 

 end of the articular, while the insertion of the anterior pterygoid seems 

 to have been in the usual position, wrapping around the end of the man- 

 dible as in other reptiles. This last muscle may have been reduced, but 

 its insertion areas on the articular and prearticular seem evident. 



HOMOLOGIES OF THE JAW MUSCLES IN VEETEBEATES 



(Tables I-V) 

 Introduction 



The first attempts to homologize the jaw muscles in the different groups 

 of vertebrates were based purely upon similarities of function and posi- 

 tion ; and while the work was useful and gave a start in the right direc- 

 tion, it was not altogether reliable. The older writers quickly took up the 

 innervation, as its importance in comparative anatomy became known, 

 and applied it to the determination of the muscles. Thus a much more 

 certain classification arose. Most of the work has been on restricted 

 groups. Yetter in his works on the elasmobranchs and other fishes gave 

 for the muscles of these groups the names that are still used. He divided 

 the muscles up into groups according to their innervation and position 

 and gave the best basis for the classification of the piscine musculature. 



One of the few papers dealing with the entire vertebrate group is a 

 dissertation by Dr. Ernst Teutleben, published in 1874 under the title, 

 Ueber Kaumuskeln und Kaumechanismus bei den Wirbelthiere. He ex- 

 amined a series of vertebrates and gave a very good description, for the 



