513 



the adult type. In the embryonic pig, Kingsley has described what 

 he considers to be traces of an old articulation between the incus and 

 the auditory capsule. In Perameles, this condition of affairs is 

 strikingly shown. The body of the incus is opposed to a flat surface 

 formed at the junction of the supra-auditory cartilage and the capsule, 

 while the short crus of the incus crosses over the "articular" pro- 

 jection of the capsule and runs back in a pit in the capsule between 

 it and the squamosal — a condition which has been figured by Fuchs 

 in the oat. 



The imperforate stapes presents no points of interest for the 

 purposes of this note. 



In the model the tympanic bone is of the greatest interest. No 

 cartilage can be detected in its development. It lies below and rather 

 to the outside of Meckel's cartilage. Anteriorly, it starts as a rod 

 lying outside and below Meckel, and between it and the dentary. 

 It runs forward for a short distance, increasing in girth as it does 

 so, and then widely forks. The plane of the forks is vertical and the 

 upper branch runs directly backwards at the outside of Meckel. 

 The lower fork drops at first at rather more than a right angle with 

 the upper, and then below curves forwards. 



The two forks carry between them the tympanic membrane, 

 which from the earliest stages is inclined only slightly out of the 

 vertical. This does not support the view that the membrane is pri- 

 marily oblique, as it is in the adult of many "primitive" mammals. 



That the tympanic is a tri-radiate bone is not altogether new. 

 Gadow demonstrated this in the foetal Orycteropus, but he suppo- 

 sed the posterior limb to represent an old articulation with the jaw 

 instead of being the remains of an actual jaw bone. There are traces 

 of this posterior limb of the tympanic in several primitive adult 

 mammals. 



The chief interest of the model lies in a comparison with the 

 condition in the Therapsids, the jaws of which have been worked out 

 in detail by Watson. In Figs. 3 — 4 the outer and inner aspects of the 

 posterior part of the jaw of a Cynodont are represented for compari- 

 son. The figures have been drawn from specimens of Gompho- 

 gnathus and Cynognathus in the British Museum (Natural History). 

 In these forms the dentary is a large thin bone curved exactly like 

 that in the foetal Perameles, and carrying loosely within its lower 

 curve the rest of the bones of the lower jaw. Those with which this 



Anat. Anz. Bd. 43. Aufsätze. 33 



