298 COMPARATIVE ZOOLOGY OF CHORDATES 



and the tympanic membrane, thus forming part of the chain 

 of three auditory ossicles which is characteristic of mammals. 

 The columella auris (or hyomandibula), pierced by the 

 stapedial artery, comes to look like a stirrup and is hence called 

 the stapes ; the quadrate is now called the incus, and the 

 articular becomes known as the malleus. At the same time 

 the angular becomes converted into the tympanic bulla (also 

 peculiar to mammals) and the supra-angular is represented by 

 the processus Folii ; the remaining bones of the Theromorph 

 reptiles have disappeared. It is a striking fact that the 



Fig. 151. — Diagrammatic views showing the transition from the reptilian 

 to the mammalian method of articulation of the lower jaw : A, reptile ; 

 B, mammal. 



a, angular ; art, articular ; ca, columella auris ; d, dentary ; t, incus 

 (quadrate) ; m, malleus (articular) ; Mc, Meckel's cartilage ; pt, processus 

 Folii (supra-angular) ; q, quadrate ; s, squamosal ; sa, supra-angular ; st, 

 stapes (columella auris) ; /, tympanic (angular). 



mammalian ear is associated with bones which in the ancestors 

 served to form the articulation between the upper and lower 

 jaws. The remarkable change of function which these bones 

 have undergone is, however, less remarkable than would 

 appear at first sight, for their essential feature is that they 

 remain articulated to one another, and so are able to transmit 

 the vibrations of sound. The columella auris is pierced by 

 an artery and resembles the stapes in certain lizards and 

 Gymnophiona, and in the latter group of animals it may be 

 connected with the quadrate. There is therefore no radical 

 innovation in the fact that the incus articulates with the stapes. 



