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Show 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 reptilian quadrate is very questionable. 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 embryology or palaeontology, and the support which it seems to get from comparative anatomy is in my opinion the result of misinterpretation 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 mammalian 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 articulation, 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, 0, 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 |