404 



laginous foundation of the primitive ring and tympanum) is the modi- 

 fied quadrato-jugal. This is not absolutely impossible, but it is un- 

 necessary and very unlikely. The quadrato-jugal is often lost in 

 closely allied Reptiles, for instance in various genera of Chelonia (in 

 which, by the way, the whole quadrate is shaped into a huge tym- 

 panic ring); it is also absent in various Theromorpha. Moreover, 

 whenever it is still typically developed, e. g. in Lacertilia, it is mark- 

 edly three-armed, rather like the os tympanicum of low Mammals, with 

 a shaft eventually attached to the squamosal and to the mandible, and 

 with a third, free arm, but this free arm of the quadrato-jugal points 

 forwards, not backwards; the articulation of the quadrato-jugal with 

 the mandible is effected by the most lateral, external portion of the 

 articulare, or even with the lateral splint bone, the supra -angulare. 

 The primitive os tympanicum on the other hand is articulated with 

 the innermost part of the articulare. Of course those, who favour 

 the quadrate = incus view, fight shy of a survey of the features of 

 the OS tympanicum. Macroscopic dissection, the observation of actual 

 facts, leave alone the examination of a large series of different genera, 

 are discarded for ontogenetic, mostly cenogenetic vagaries. Peters 

 had interpreted the distal knob of the shaft of the tympanic of Echidna 

 as the old articulation with the mandible and I have further elaborated 

 his sagacious observation by suggesting that this knob of the tympanic 

 originally articulated with that part of the mandible, which in Mar- 

 supials is known as its inner angle. This I am now able to prove. 

 Mr. Oldfield Thomas has kindly permitted me to examine two very 

 young specimens of Orycteropus, probably foetuses, and a very 



immature skull. Another skull has 

 been figured in Phil. Trans, op. cit. 

 PI. 73, Fig. 28. In the two youngest 

 of these skulls the conditions are as 

 follows. See Fig. 3. In the natural, 

 undisturbed state, the knob of the 

 tympanic almost touches the pro- 

 cess I of the mandible, with which 

 it is not connected by ligaments. 

 Immediately above this vanishing 

 mandibulo- tympanic joint, and me- 

 dially from it, passes the carti- 

 laginous continuation of the malleus into the Meckelian cartilage of 

 the mandible. The still open space will of course become Glaser's 

 fissure. The distal knob of the tympanic or quadrate is much sharper 



Fig. 3. Youug Orycteropus. 



