ADAMS, PHYLOGENY OF THE JAW MUSCLES 61 



that they arose as muscles for contracting and dilating the gill openings, 

 drawing in water containing food and oxygen, and finally that they were 

 used in snapping at prey (Gregory, 1915). The predatory habits of the 

 primitive fishes were responsible, it is believed, for the change of the 

 gill muscles into true jaw muscles. Even in mammals the muscles are 

 still intimately related with the branchial arches, with the tongue and 

 even with the ear. The point of attachment and the function of a mus- 

 cle must then be considered as one of the available criteria of homology. 

 This should always be considered when the history of the bone to which 

 the muscle is attached has been thoroughly studied through a number 

 of classes and when also its developmental history is certain. 



Neomorphs have often arisen as slips from some of the muscles; 

 muscles frequently have split up into slips that eventually have become 

 separate and taken a part of the parent nerve with them as in the ptery- 

 goids of the mammals. We have numerous examples of this splitting: 

 for example, the derivation of the anterior belly of the digastric of mam- 

 mals from the mylohyoid, or the subdivision of the "adductor mass" 

 into numerous slips in Amia. If this splitting is carried further and 

 the slips separate, it becomes correspondingly more difficult to trace their 

 homology. 



Acknowledgments 



This work has been carried on in the Department of Vertebrate Palaeon- 

 tology at the American Museum of Natural History, under the general 

 direction of Professor W. K. Gregory, from whom the author has con- 

 stantly received critical suggestions and helpful advice. I am also grate- 

 ful to Professor E. C. Osburn for the loan of private material used in a 

 part of the work on the teleosts ; and to Mr. J. T. Nichols for other speci- 

 mens of fish needed in the teleost series. To Dr. L. Hussakof my thanks 

 are due for his placing at my disposal numerous specimens from the col- 

 lection of Arthrodira in the Department of Ichthyology. Finally, in 

 common with many other investigators, I am mindful of a larger debt to 

 the American Museum of Natural History for liberty to use its extensive 

 resources. 



A series of 26 existing types of vertebrates has been dissected as fol- 

 lows: Elasmobranchii 1, Chondrostei 2, Holostei 1, Teleosti 3, Crossop- 

 terygii 1, Dipnoi 1, Urodela 3, Anura 1, Chelonia 1, Ehyncocephalia 1, 

 Lacertilia 2, Crocodilia 1, Aves 1, Mammalia 7. In each case special 

 attention has been paid to the innervation of the muscles as a guide to 

 homologies. By means of these data, and of the principles that became 

 apparent as the work proceeded, reconstructions of the jaw musculature 



