No. 2304. REPTILIAN CHARACTERS IN MAMMALS— WORTH AN. 27 



the tympanic cavitj'-, namely the incus. The articular of the reptilian 

 mandible has lost all connection with the lower jaw and has become 

 the malleus of the mammalian ossicular chain. (2) The auditory 

 chain of bones of the mammalian tympanum have been derived 

 from and are strictly homologous with a similar chain of bones in the 

 reptilian or batrachian skull frequently found as an undifferentiated 

 rod of bone, the collumella auris. The quadrate has disappeared, 

 having become either the tympanic (Gadow), the inter-articular 

 fibro-cartilage of the glenoid cavity (Broom), or incorporated with 

 the squamosal (Cope and Bam). 



When one studies the quadrate in a large series of reptiles and 

 birds he can not well avoid being struck with the superficial resem- 

 blances of this bone to the mammalian incus. This is heightened by 

 the peculiar manner in which the quadrate articulates in birds and 

 is attached to the side wall of the brain case by a ball-and-socket 

 joint, not dissimilar to the way in which the short process of the 

 incus is received into the fossa incudis of many mammals. Then, 

 again, there is the peculiar and wholly characteristic double saddle- 

 shaped articulation of the incus and malleus of the mammalian 

 tympanum, which at once recalls the articulation between the 

 quadrate and articular of the reptilian jaw, and lastly the relatively 

 large size of the incus and malleus in certain of the lower forms of 

 the mammalia, notably the monoti-emes. Another supposed fact 

 which has been looked upon as having an important bearing upon 

 the question and used in support of this hypothesis is the assumed 

 complete absence of any lepresentative of an articular element in 

 the mandible of the mammal. This is stated by Gregory^ as 

 follows : 



" In order to substantiate the conclusion that the mandibulo-squa- 

 mosal joint in mammals is a wholly new structure, into which the 

 quadrate and articular did not enter, we recall the facts (1) that 

 embryological research gives no warrant for the belief that the 

 mammalian jaw is composed of more than one element (excej)t for 

 the occasional vestiges of a splenial) ; (2) that all the oldest loiown 

 mammalian jaws, from the Triassic, Jurassic, and Basal Eocene, 

 never show any trace of sutures; (3) that in the CSmodonts the broad 

 ascending ramus or corono-condylar region appears from Broom's 

 researches to be a part of the dentary." 



For the sake of brevitj^ this theory may be called the transposition 

 theory of the quadrate and articular. 



The other theory of the fate of the reptilian quadrate in the mam- 

 malian skull assumes that it has gradually disappeared ^^athout 

 having entered the tympanic chain; that the ossicvla auditus of the 

 mammahan tympanic cavity have been derived directly from a 



1 The Orders of Mammals, Bull. Amer. Mus. Nat. Hist., Feb., 1910, p. 138. 



