172 



CHORDATE ANATOMY 



connexion between the ear-drum and the inner sensory ear, the columella 

 or stapes. The hyoid cartilage forms the basis of attachment of the 

 tongue muscles. With the disappearance of the gills as functional organs 

 in land amphibians, the remaining visceral cartilages become further 

 reduced and associated with the larynx. 



Some advance towards the mammaUan visceral skeleton appears in 

 living reptiles. The teeth both in upper and lower jaws become lodged 

 in alveolar sockets in two membrane bones, the premaxillary and maxillary 

 of the upper jaw and the dentary or mandibular of the lower. These 



- SURANGULAR_. 

 PREARTIOJLAR.. 



ARTICULAR 



DIMETRODON. 



,Dgn"ARY 

 INNER ASPECT symphysis 



.SUR ANGULAR 

 PREARTICULAR 



OUTER ASPECT. 



'ARTICULAR 

 SCYMNOSUCHUS 



SURANGUI 

 PREARTICULAR 



CYNOGNATHUS, 



S* 



Fig. 162. — Jaws of Tertiary reptiles viewed from their inner and outer aspects. 

 Dimetrodon belongs to the lower Tertiary, while Scymnosvichus and Cynognathus 

 belong respectively to the middle and upper Tertiary. A notable increase in the size 

 of the dentary and relative diminution in the size of the articular element is seen. ^ Thus 

 the articular was set free to become the malleus of the ear. By similar changes in the 

 upper jaw the squamosal bone replaced the quadrate, which then became the incus. 

 (Redrawn after D. M. S. Watson.) 



membrane bones, however, do not extend as far as the articulation of the 

 jaw which, as in amphibians, is between the quadrate of the upper jaw 

 and the articular of the lower. 



In mammals a new articulation of the jaw is formed between two 

 dermal bones, the dentary of the lower jaw and the squamosal of the brain- 

 case. How such a change could occur while the jaw was still functioning 

 has been one of the vexed questions in vertebrate morphology. Thanks, 

 however, to recent discoveries of fossil reptiles which bridge the gulf 

 between reptiles and mammals, the way in which the shift was effected is 

 now fairly clear. See Fig. 162. 



D. M. S. Watson and others have found that in Tertiary reptiles 

 changes in the articulation of the jaw occurred which involved reduction 



