CYNODONT REPTILES QOMPHOGNATHUS AND CYNOGNATHUS. 205 



except the one in question, which alone has to retain the old 

 geneiic name of Gomphognathiis* . 



Having tinished the comparison between the lower jaws of 

 Gomphognathus and Cynognathus, I will add here some general 

 remarks concerning the origin of the mandibular articulation and 

 the origin of the Ossicula auditus in Mammals, inasmuch as these 

 two veiy closely connected problems receive new light from oar 

 comparison. 



That the mandibular articulation in the most primitive mammals 

 (the hypotheticid order Promammalia of Gregory — comp. its 

 definition in Gregory, 1908, p. 164) was a double one, our 

 Gomphognathus-iikuW puts this point beyond question, as it shows 

 that in this advanced Theriodont the dentary almost if not 

 actually reached the squamosal. That the dentary articulation 

 was situated in the same plane with the articular articulation, and 

 vot in front, is also a point put beyond question. That the two 

 articulations have worked closely together and practical 1}' as one 

 articulation, is also very probable (on these three points in 

 primitive Mammals comp. Gregor}"^, 1908, pp. 135-138). 



Our Gom])hognathus-'s^\\\\ shows also the way in which the 

 cotylus and condylus of the dentary articulation in Mammals 

 have probably arisen. As the articular surface of the dentary 

 situated on the squamosal is a smooth plane surface in our 

 specimen, so we have to suppose that also the corresponding 

 surface on the condylar prolongation of the dentary was a smooth 

 plane surface. Consequently we may suppose that this latter 

 surface was a mechanical lesult of the latei-al movements of the 

 lower jaw, through which the hind end of the dentary {%. e. the 

 condylar prolongation of its ascending process) was brought in 

 contact with the squamosal. The primitive condition of the 

 dentary articulation in Mammals would, according to this 

 supposition, be a simple syndesmosis, as we find svich a syndes- 

 mosis secondarily in Tatusia hyhricla among living forms. The 

 different forms of the mandibular articulation in higher Mammals 

 would then be considered as further mechanical results of jaw- 

 movements according to the mechanical theory of normal articu- 

 lations of Fick t, Tornier j, and Cope §. 



Still greater is the importance of our specimen in respect to 



* Between the skulls of Cynognathus on the one side and the skulls of Diade- 

 modon (and perhaps also of Gomphognathus) on the other, 1 find an essential 

 difference in a separate ossification that exists in front of the epipter3'goid and below 

 the postfrontal bone in the British Museum specimen R 3587 of Diademodon- 

 skull (described by Watson, 1911, who has not recognized its separate nature), an 

 ossification that 1 suppose to be an incipient orbitosphenoid. The front edge of 

 the epipterygoid bone in Cynognathus crateronotus is damaged, but the under 

 surface of the postfrontal bone is so smooth, that a correspondmg separate orbito- 

 sphenoidal ossification is a very improbable one (comp. fig. 6 in Watson, 1911, p. 300, 

 with fig. 6, p. 76, in Seeley (2), 1895). 



f Comp. R. Fick, " Ueber die Form der Gelenkflachen " in ' Arcliiv fiir Anatomie 

 und Physiologic,' 1890, p. 391. 



X Comp. J. Tornier, "Das Entstehen der Gelenkformen " in ' Archiv fiir Ent- 

 wickelungsniechanik,' 1897, pp. 124-158, 224-268, 307-346. 



§ Comp. E. Cope, 'Primary Factors of Organic Evolution,' 1904, p. 283 seq. 



