OCR Text |
Show 1904.] OF THE THERIODONT MANDIBLE. 491 part of the squamosal. The whole of the mandibular articulation is formed by the articular bone. The exact size of the bone is not seen in the specimen as it is largely overlapped by the angular. The angular is seen between the posterior and under border of the dentary and the articular bone. As seen in the specimen, it appears to be a thin flake of bone lying on the outer side of the articular. A good deal of the bone has, however, been removed while the specimen was being developed, and the angular should extend as far back as the groove which is seen on the outer side of the posterior part of the left jaw. The surangular is not very well seen in this specimen. The splenial, except on the fractured surface at the front of the specimen, is also imperfectly displayed. The quadrate is well shown on the left side and fairly well on the right. In the development of the specimen this region has been to some extent ground down, but this has resulted in the relations of the quadrate to the articular and to the squamosal being well shown. In fig. 1 (PI. X X X V . ) the quadrate is seen inter - digitating with the squamosal, almost exactly as in Cynognathus crateronotus. On the outer side of the articulation, the articular comes almost in contact with the squamosal, and it seems not improbable that the articulation of the mandible is here directly with the squamosal. On the right side, only a part of the middle and of the inner end of the quadrate is seen. Passing directly inwards from the inner end of the quadrate there is seen on both sides a very remarkable elongated bone. As it is now seen in the specimen, it appears to be a somewhat cylindrical bone, hollow in the centre, and from which part of the thin wall has been removed during the removal of the matrix. Its outer end evidently articulates with the quadrate, and tho inner end with a part of what may be the periotic. Seeley recognises that this bone is evidently the homologue of the dumbbell- like bone in the similar situation in Dicynodon-the bone which, when dealing with the Anomodonts, he believed to be the malleus. In his paper on Cynognathus (1895) he says: "I now incline to regard it as a rudimentary straight cochlea." In describing the skull of Udenodon in 1901 (2), I expressed the opinion that the bone in the Dicynodonts was the homologue of the mammalian tympanic, and I still incline to this opinion. In Udenodon the bone is solid, so that it cannot have lodged any part of the inner ear. In Dicynodon the columella auris lies in the hollow formed by the bone and the exoccipital. In Cynognathus there is likewise a hollow between the exoccipital ("opisth-otic " of Seeley) and this supposed tympanic, and in this hollow there he one or two tiny bones or portions of bones which may represent the auditory ossicles. Two other specimens probably of one individual, and belonging to a species of Cynognathus (either C. berryi or a new species). |