62 ANNALS NEW YORK ACADEMY OF SCIENCES 



were attempted in the following series of extinct forms: Dinichthys 

 (Arthrodira), Eryops (Temnospondyli), Labidosaurus (Cotylosauria), 

 Tyrannosaurvs (Theropocla), Cynognathus (Cynodontia). 



THE JAW MUSCLES IN RECENT VERTEBRATES 



Pisces 



acanthias 

 Plate I, Figs. 1, 2 



The elasmobranchs as primitive gnathostomes are far superior in rank 

 to the pregnathostome stage represented by the ostracoderms, for the first 

 two visceral arches have already taken their places as the future supports 

 of the jaws and hyoid arch. For many geological ages the teeming multi- 

 tudes of pregnathostomes no doubt tried to develop a perfected form of 

 jaws, but up to the appearance of the elasmobranchs the attempts to de- 

 velop something besides a suctorial mouth were hardly satisfactory. 



Somewhere in early Palaeozoic times the vertebrates acquired the car- 

 tilaginous jaws, for the acanthodians as far back as the upper Silurian 

 had already developed the type of jaw that is to continue through the 

 rest of the vertebrate series. The cartilages of the skull and body in the 

 elasmobranch give a fairly firm and stable attachment for the muscles. 

 The great advance has been in the change of function of the two anterior 

 visceral arches. The first two arches of the visceral series have lost their 

 function as supports for the gills, and form the cartilaginous structure 

 that is to become the jaw and hyoid apparatus of the gnathostomes. The 

 first arch has grown forward under the brain case and has attached itself 

 to the ethmoid region at the anterior end and to the hyomandibular re- 

 gion posteriorly. The palato-quadrate bar is continuous, forming a long 

 bar extending from the anterior end to the posterior part of the side of 

 the skull. 



The teeth have already developed in the sharks. The denticle-covered 

 skin has been drawn into the mouth and by growing together or enlarg- 

 ing the denticles have developed into fairly efficient teeth. 



The jaw system in the shark is just the opposite of that in Dinichthys. 

 The maxillary region is securely fastened to the skull, although it may be 

 movable, while the mandible is the movable agent with the articulation 

 in the quadrate region of the palato-quadrate bar. This mandible in the 

 shark is capable of movement only in one direction. There is no side 

 movement in these forms as there is no muscular system to operate it. 



