ANCESTRY OF THE MAMMALIA 



43 



Of these the dentary hears the teeth; the angular and surangu- 

 lar lie near the hinder end; the articular forms a joint with the 

 quadrate bone of the skull, a bone which has disappeared as 

 such in the Mammal ; the splenial lies upon the inner side of the 

 dentary. Not only does the mammalian mandible differ from 

 that of the Reptile in consisting of a single element but the 



Fig. 8. — Morphology of the Mandible and Auditory Ossicles (after Gregory,* Orders 

 of Mammals. Bull. Am. Mus. Xat. Hist., 1910, vol. 27, fig. 2, p. 124). 



A. Mandible of a Permian Therocephalian (Lycosuchus vanderrieti, after Broom). 

 Ang., angular; Ar., articular; D., dentary; p. an., processus angularis; Qu.. quadrate; 



S. ang., surangular; Sq., squamosal; z. So., zygomatic process of squamosal. 



B. Skull and mandible of Cynognathus'platyceps (composed from figures by Broom). 

 Abbreviations as in A. Mk., Meckel's cartilage; P. art., prearticular (Williston, i. e., 



the dermal medial extension of the articular) ; St.. stapes. 



C. Developing mandible and auditory ossicles of Kangaroo (Macropus, after Bens- 

 ley). 



Abbreviations as in foregoing. In., incus; Ml., malleus; o.ant. processus anterior 

 mallei; Pr. ar., prearticular; St., stapes (turned back); Ty., Tympanic ring. 



articulation of the bone with the skull is essentially dissimilar. 

 In the Mammal the articular and quadrate disappear as such, 



*This illustration has been modified by Dr. Gregory since the publication of his 

 monograph, The Orders of Mammals, from which the figure was originally taken. By 

 kind permission of Dr. Gregory the most recent modification of the figure is here re- 

 produced. 



