1904.] OF THE THERIODONT MANDIBLE. 495 



exist in the intermediate stage when the quadrate was being 

 modified into the incus. " Let us examine," he says, " what 

 terrible intermediate stages this would imply. A lever-bar, the 

 mandible, supported at two places, one behind the other in the 

 long axis. Such a thing would not be able to move ; the animal 

 could not use its jaws, and this intermediate stage would also 

 imply the giving up of hearing through the tympanum, and 

 through the columellar or stapedial apparatus, until the quadrate, 

 relieved of its suspensorial function, had slipped in with the 

 articulate [malleus] and had re-established the connection between 

 stapes and tympanum ! " 



While there can be little doubt that Gadow is right in holding 

 that the mammalian auditory apparatus is homologous with the 

 auditory apparatus of reptiles, his view that the mammalian 

 tympanic corresponds to the i-eptilian quadrate is very question- 

 able. It is impossible in the present paper to enter into the 

 discussion of the question, but it may be pointed out that the 

 view, though free from the fatal objections that can be urged 

 against the " incus " view, receives no support from either embry- 

 ology or palaeontology, and the support which it seems to get from 

 comparative anatomy is in my opinion the result of misinter- 

 pretation of one or two of the facts. 



The examination of the Theriodont jaw and of its mode of 

 articulation shows that the condition is already so nearly mam- 

 malian that only a very slight modification, and that very easily 

 understood, is required to convert the Theriodont jaw into that 

 of the mammal. 



In text-figure 100, A, p. 496, we have a representation of the 

 jaw of a Therocephalian, with the quadrate and squamosal. The 

 large size of the angular is well shown and also the well- 

 developed quadrate. The mouth is supposed to be open. 



Text-figure 100, B, p. 496, is a view of the jaw of a Theriodont 

 with the squamosal and quadrate. In this figure also the mouth is 

 open to show the whole of the dentary. The small relative sizes 

 of the articular and angular are manifest, but the most striking 

 feature is the great development of the dentary. It will be seen 

 that though the dentary does not form part of the ai'ticu- 

 lation, very little modification would be required to convert the 

 jaw into one in which the dentary formed the articulation. When 

 the articular surface was transferred to the dentary, the articular 

 element and with it the angular would rapidly degenerate. 



Let us now examine the condition of the jaw in a young 

 mammal. Meckel's cartilage is continuous with the malleus, but as 

 the malleus is a hyomandibular element it may be removed, and 

 we have then, as in text-figure 100, C, in addition to the dentary, 

 the posterior portion of Meckel's cartilage. • It will be observed 

 that this portion of the cartilage is in close relationship with the 

 condyle. Gadow figures Meckel's cartilage as if it entered the jaw 

 near the angle, but in all the mammalian embryos that I have 

 examined the cartilage lies by the side of the condyle, and where 



