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Show 1904.] OF THE THERIODONT MANDIBLE. 497 Meckel's cartilage as the degenerate remains of the articular. When the quadrate became rudimentary and the dentary took up the articulation, Meckel's cartilage would be brought into hue with the malleus, and the two elements, though morphologically quite distinct, ever afterwards would develop as one continuous structure. In the development of Trichosurus we find an analogous case of two distinct structures developing as one. Here the coracoids and sternum develop as a single cartilage, though the sternum is a costal structure and not part of the shoulder-girdle. There remains to be considered the question of the fate of the quadrate. In the Theriodont it is a small bone fixed to the front and lower end of the descending process of the squamosal. When we look in this region in the mammal, we find either the mandible directly articulating with the squamosal, or indirectly owing to the intervention of a thin plate of cartilage. In the large majority of mammals this small cartilage-the interarticular cartilage or meniscus-is present: in only one or two-the Monotremes, Dasyurus, Dasypus-is it absent as a cartilage : occasionally it is ossified, as in Pedetes. Some years ago, I suggested the possibility of this cartilage being the quadrate (9), and all the palaeontological evidence which has since been discovered seems but the more strongly to confirm the view. It has been said that if this were so we should expect the cartilage to be most strongly developed in the Monotremes-just where it seems to be entirely absent. It must be remembered, however, that the Monotremes are extremely specialised and in some respects very degenerate. The vertebrae other than the axis are much less like those of the Theriodonts than are those of the Eutheria: in the carpus the os centrale is absent, though still retained in many higher forms; and the mandible in both Monotremes extremely degenerate and very unlike that of the Theriodont, while in the Eutherians the lower jaw resembles the Theriodont jaw very considerably. It does not seem therefore so very strange that the quadrate by taking on a special function in the mandibular joint should be retained in the higher forms though lost in the lower. On the other hand, it is quite possible that the quadrate is entirely absent in all mammals; yet the presence of a cartilaginous structure in a situation exactly corresponding to that of the quadrate in Theriodonts seems strongly to favour the view that in the meniscus we have the modified equivalent of the reptilian quadrate. REFERENCES TO LITERATURE. (1) H. G. SEELEY, "On the Skeleton in new Cynodontia from the Karroo Rocks." Phil. Trans, vol. 186 (1896) B, p. 59. (2) R. BROOM, "On the Structure and Affinities of Udenodon." Proc. Zool. Soc. 1901, vol. ii. p. 162. |
