405 



and more pronounced than in the immature, and it becomes quite 

 abolished in the adult. We also note that the annulus is at first 

 scarcely more than a semiring, that it articulates with the squamosum 

 only by the upper, proximal end of the shaft, that this is not merely 

 a lean-to, but an elaborate articulation, that the semiring is still com- 

 pletely separated from the alisphenoid, periotic and lateral occipital, 

 and lastly that the posttympanic end of the semiring is still free. 

 Later on this reaches the corner between squamosal and opisthotic, 

 resting laterally upon a shelflike process of the opisthotic, and to 

 the blunt end of this process is attached the cartilage of the stylohyal. 



The process I of the mandible is the homologue of the inner or 

 inverted angle of the underjaw, not of course the articulare itself nor 

 even part of the Meckelian cartilage, but it is now formed by part 

 of the dentale, which in Mammals envelopes and forms the whole jaw, 

 after the abolishment of the other splint-bones. It is an unimportant 

 modification that this process I is curved inwards in most Marsupials ; 

 it is a change brought about by the internal pterygoid muscle. In 

 the triassic Amphitherium it still extends straight backwards. In 

 Myrraecobius that part, which is undoubtedly the „inner angle" is 

 long, still straight, and scarcely curved inwards. Even in the Rodent 

 D 1 i c h 1 i s and in Chinchilla there exists such a process, huge and 

 concave, i. e. inverted. Parker figures it well in the young of T a t u s i a , 

 Dasypus, Talpa,Centetes etc. in Phil. Trans. 1885 there labelled 

 ag. p. i. e. angulus posterior, unmistakable traces of it are still 

 present in Echidna, in spite of the much reduced jaw, and in Or- 

 nithorhynchus, but in either case far removed from the tympanic, 

 at least in the adult. In most other Mammalia it seems to have dis- 

 appeared. The same inner angle exists in many Birds. The homology 

 of this angle is beyond doubt in the various animals. Its topographic 

 landmark is perfectly well fixed by the extracolumellar-mandibular, or 

 by the malleo-mandibular string of cartilage. 



But how, and why, is this angle formed? Because the joint is 

 shifting. The quadrato-mandibular joint is not formed simply by one 

 cup and ball, but it is complex already in many Reptiles. In Chelonia, 

 for instance in Trionyx hurura the quadrate bears a median, flat 

 surface which plays upon the flat articulare, and a lateral convexity 

 which fits into a deep and wide cavity which is formed entirely by the 

 supraangulare. In other Reptiles there are two knobs of the quadrate 

 fitting into two cups of the mandible and these cups are divided by 

 a ridge, e. g. in Sphenodon. In many Birds the joint has become 

 still more complicated by a third, still more lateral, cup and ball. 



