ADAMS, PHYLOGENY OF THE JAW MUSCLES 



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Figure 4 



Dermal jaw bones of various arthrodires, lateral surfaces ; from specimens in the 

 American Museum of Natural History. 



The dermal lower jaws of arthrodires were probably not connected with the upper jaw 

 by means of quadrate and articular cartilages as in true fishes, but may have been fas- 

 tened in the thick dermis surrounding the oral cavity. 



1. Stenognathus gracilis. — This represents a long-jawed specialization from a Dinich- 

 tJiys-like type. 



2. Dinichthys intermedins. — The shearing portion of the jaw is raised above the plane 

 of the horizontal ramus which was probably embedded in the thick dermis. 



3. Diplognathus miraoilis. — A very peculiar offshoot of the coccosteid type. The sym- 

 physeal border of the lower jaw bears tooth-like projections which apparently indicate 

 that each jaw plate could be twisted on the long axis in a manner impossible in true 

 fishes (Dean). 



4. Dinichthys curtus. 



5. Dinichthys intermedins. — To the upper end of this specimen on the outer side is 

 attached a triangular bone which may have served for the insertion of muscles and 

 fascia movably connecting the mandible with the inner side of the skull, somewhat as 

 the scapula of mammals is connected with the body. 



