142 Bulletin American Museum of Natural History. [Vol. XXVIl, 



derived from streptostylic reptiles of the type represented in the Lacertilia; 

 and strong anatomical and pali^ontological evidence reviewed above shows 

 that the squamoso-mandibular joint in mammals is, in fact, a neomorph. 



An attempt was made to demonstrate (])p. 136, 140) that the ai^plication 

 of the above mentioned "ciuadrate-incus" doctrine to the conditions in the 

 Cynodontia might ex}dain to a certain degree the supposed transformation 

 of reduced jaw elements into accessorij auditory ossicles. Upon this assump- 

 tion these structures only very gradually gave up the suspensorial function, 

 dwindled greatly in size and began to come into functional relations with the 

 stapes-extracolumella, possibly serving at first merely as a point of support 

 for the middle part of this jointed column (Fig. 7, B). But until every 

 mechanical and morphological stage in the progressively intimate relations 

 of the primary and secondary ossicles to each other and to the muscles be 

 clearly conceived and supported by well founded analogies, the quadrate- 

 incus, articular- malleus theory must remain somewhat doubtful. In its 

 defense it might be said that the functional integration of the stapes-extracol- 

 umella with the vestigial quadrate and incus was but one incident in the 

 wonderful metamorphosis of saurians into mammals, a process which 

 involved the transformation of scales of some sort into hairs, of skin glands 

 into milk-secreting organs. But it must be admitted that in these cases the 

 chance of function was less radical and the intermediate steps more easily 

 conceived than in the supposed case now under consideration. 



The ([uadrate-incus theory may not be seriously hampered by the partial 

 fixity of the (juadrate in Cynodonts because a secondary increase in the 

 mobility of the ((uadrate is quite conceivable under the conditions assumed 

 (p. 139) ; but a more serious objection is the fact that Kingsley's view of the 

 separate origin of the manubrium of the malleus has not been confirmed by 

 subsequent research, and therefore the opposing arguments for homologizing 

 the malleus with the extracolumella rather than with the articular have not 

 been fully met. 



Turning now to the opposing theory, held notably by Gadow and 

 Broom, we note that it makes no demand for a very hardly conceived and 

 elaborate change of function, since it regards the tympanic membrane and 

 the auditory ossicles of the lizard and crocodile as strictly homologous with 

 those of mammals; and it assumes only that: 



extracolumella = malleus, 



suprastapedial = incus, 



stapes = stapes. 



On the other hand the weak points of this theory appear to be as follows: 



(1) The connection between the malleus and Meckel's cartilage, which 

 is universal in mammalian embryos of all orders, must be assumed to be 



