AUDITORY REGION IN DICYNODON. 425 



quailrate, articular, and angular degenei-ated, and might have 

 been lost had not the attachment of the stapes to the quadrate 

 compelled them to take on an auditory function. 



The tracing of the steps by which the articular became the 

 malleus and how the manubrium was formed is a matter of some 

 ditKculty in our ignorance of the position of the tympanic 

 membrane, but we may assume as very prolnible that soon after 

 the dentary-squamosal joint was formed, the angular and articular 

 rapidly degenerated and ceased to move with the jaw. The 

 tympanic membrane, which possibly in the Cynodonts lay behind 

 the plane of the articulation, probably in the early mammals 

 became shifted more forwards and inwai'ds, and the articular came 

 to support the membrane by the development of the manubrium, 

 while the angular also took part in the support of the membrane 

 and became the tympanic bone. Gaupp is probably correct in 

 regarding the goniale as not the homologue of the angular, but a 

 distinct membrane-bone closely associated with the articular. 



In the series of six diagrams given (text-fig. 53, A-F), the 

 probable course of the evolution of the auditory ossicles is shown 

 from the Cynodont type to the type found in the mammal. 

 Though the first figure (A) represents the Anomodont type, this 

 must not be looked upon as on the direct mammalian line, but as 

 a specialised modification of the ancestral type. In fig. B we have 

 represented the Cynodont type as seen in Cynognathus. The 

 articulation is formed by the articular hinging on the quadrate 

 and squamosal. The dentary almost reaches the joint. Fig. C 

 represents the probable condition in the later Cynodont type 

 when the dentary came to take part in the articulation. Fig, D 

 represents the later stage when the quadrate has completely 

 slipped out from the joint and the articular and angular have 

 degenerated and no longer move with the dentary. In fig. E we 

 have represented what was the probable condition when the 

 articular and angular first became specialised for their auditory 

 function. This is almost the Monotreme condition. Fig. F 

 represents the condition in the t^qiical higher mammal. 



EXPLANATION OF PLATE LVL 



A.S.C. Asceiulin<!; semicircular canal ; B.O. Basioccipital ; B.S. Basisplienoid ; 

 Car. Carotid artery; J3.0. Exoccipital ; E.Pt. Epipterygoid ? ; F.O. Fenestra 

 ovalis ; F.3I. Foramen magnum; H.S.C. Horizontal semicircular canal: Ju. 

 Jugal; O.C. Occipital condyle; P.S.C. Posterior semicircular canal; Ft. Ptery- 

 goid; Q.J. Quadrato-jugal ; Qii. Quadrate; Sq. Squamosal; St. Stapes; Ti/.M. 

 Tympanic membrane ; Vest. Vestibule. 



Fig. 1. Left side of decalcified brain region of small Dicynodont. Nat. size. 



2. llight side of partly decalcified brain-region of small Dicynodont. Nat. 



size. 



3. Cast of osseous labyrinth of small Dicynodont, restored from the facts 



revealed by the two sides. Nat. size. 



4. Quadrate region of Oudeiiodon Icolhei. Rediiced about one lialf. Front 



view. 



5. Quadrate region of Oudenodon koJhei. Reduced about one half. Lower 



view. 



6. Quadrate region of Oudenodon kolbei. Reduced about one half. Back 



view. 



