AUDITOKV KKOIOX I\ DKYNODOX. 423 



]\\g'.\] nliiiost to the articulation. A shallow but distinct groove 

 can he ti-aced down, pai'tly formed by the squamosal Jiiul pjirtly 

 by the exoccipital. This is evidently for the extei-nal auditory 

 canal, but how the tympanic membrane was attached or whether 

 there was one at all, the evidence does not show. Not improbnblv 

 there was a membrane attached to the exoccipital, squamosid, Mud 

 to a small part of the quadrate. 



Though the articular region is more perfectly known in the 

 Anomodonts than in other Therapsida, a, good deal is known about 

 its condition in the other groups. In the Pelycosaurs the stapes 

 is large and of a peculiar sha|)e, the cpiadrate is also large and 

 forms the whole of the articulation, and a small quadrato-jiigal is 

 present. Little is known of the condition in the Dromasauria.. 

 In the Dinocephalia the quadrate is large and foi ms the whole 

 articulation, and a, quadrato-jugal is apparently always present. 

 It is known in Delphinoynathiis, Moschnps, and TajnnocephaJus. 

 The stapes is probably very similar to ihat in the Anomodontia. 

 In the Therocephalia the quadrate is small, but forms the whole 

 articulation. Ko quadrato-jugal is known in any genus. The 

 stapes IS not well known, but appears to be large and long. In the 

 Cynodonts a quadrato-jugal is never present and the quadrate is 

 usually small. ^ In some genei-a (e. g. Cynogvathus) a considerable 

 part of the articular surface for the lower jaw is formed by the 

 squamosal, the qr.adiate forming only about three-fourths of the 

 joint. The stapes in Bauria and Cynognathus is a fairly strong cy- 

 lindrical rod : in Trirachodon it is also a long rod, but very slender. 

 In all these genera its outer end is iirmly attached to the quadrate. 

 As there are very strong reasons for believing that the mammals 

 are descended from one of the mammal-like reptiles, probably a 

 primitive Cynodont, all the information that can be obtained on the 

 structure of the auditory apjjaratus is especially welcome, because 

 of the light it may throw on the nature of the mammalian auditory 

 ossicles. And tlie certain determination of the stapes seems to 

 settle the homologies of the various elenients. Had the outer 

 end of the stapes been free and given attachment to a cai'ti- 

 laginous extrastapedial, the dispute might have gone on. But 

 there is apparently no extrastapedial, and certainly the greater 

 part of the outer end of the stapes is firmly fixed to the 

 quadrate. The similar attachment of the stapes to the incus in 

 mammals is striking ; and though I have long advocated another 

 view. I now feel forced to agree to the view that the incus is the 

 reptilian quadrate, as advocated so many years ago liy Reichert. 

 The slipping out of the quadrate from the joint is" not now a 

 very serious difficulty, for in Cynognathus we see it already partly 

 slipped out. Nor is there any very great difficulty in under- 

 standing how the new dentary-squamosal joint arose. In Cyvo- 

 gnathus, the posterior end of the dentary nearly takes part in the 

 joint, and there is no difficulty in realising the next stage in the 

 evolution, where the dentary and articular together hinged on 

 the squamosal and the quadrate. As the direct articulation of the 

 dentary on the squamosal became more firmly established, the 



