136 Bulletin American Museum of Natural History. [\o\. XXVII, 



characters from the ancestral mammals, there are several weighty objections 

 to Gaupp's diagram. First it assumes that the coronoid process of the 

 mammalian mandible is a neomorph, developed later than the mandibular 

 condyle; but there is strong palseontological evidence (Fig. 6, Xos. 1-7) 

 that the coronoid was originally very large and broad, and in reality older 

 than the condyle (see below p. 137). Secondly the diagram in (piestion as- 

 sumes that the quadrate could bend backward and forward like that of a 

 Lizard, but if the Cynodonts represent the prototypal conditions of the 

 Mammalia, then the motion of the quadrate must at first have been very 

 limited. Thirdly this process, as conceived by Gaupp on the basis of the 

 conditions in the recent Lacertilia, has been objected to by Gadow (1901, 

 p. 410) on the ground that while it was in progress the Promammal "could 

 not use its jaw and could not hear." 



I 



IV. Application to the conditions in the Cynodontia of the Theory 



THAT THE InCUS IS DERIVED FROM THE QuADRATE, THE MaLLEUS FROM 



THE Articular. 



While it might or might not have been a mechanical impossibility for 

 the mammalian relations of the lower jaw and ossicula auditus to have 

 evolved out of the conditions represented in the Lacertilia, it is at any rate 

 almost certain that in the Cynodonts a double articulation between the 

 skull and mandible is foreshadowed, if not actually realized; and yet it is 

 evident that in spite of this both the jaw and the stapes-extracolumella must 

 have been mechanically workable and useful. Broom (1904.2, ])p. 495, 496) 

 has suggested that the mammalian conditions must have been initiated by 

 the backward prolongation of the condylar process of the dentary till it 

 came in contact with the glenoid region of the squamosal (Fig. 2, B). This 

 hypothesis is strengthened by the fact that the condylar process of the 

 dentary shows an increasing backward prolongation in the following series 

 of Triassic Cynodonts and so called mammals: Bauria, Cynognathns, 

 Trirachodou, Dromatherium. 



The double articulation between jaw and skull which would thus result 

 would be homologous, so far as concerns its separate elements, with the 

 double articulation conceived by Gaupp, /. e., the new joint would be by 

 way of the condylar process and the glenoid portion of the scjuamosal, while 

 the older one would be by way of the small articulare and cjuadrate. But if 

 the conditions thus foreshadowed in the Cynodonts were actually realized, 

 these two articulations would be so close together that they would function 



