1904.] OF THE THERIODONT MANDIBLE. ' 497 



Meckel's cartilage as the degenerate remains of the articular. 

 When the quadrate became rudimentaiy and the dentary took up 

 the articulation, Meckel's cartilage would be brought into Hne 

 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 ana- 

 logous 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 tlie 

 palseontological 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 Monotremas 

 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. 



