582 Mr. D. M. S. Watson on some 



theory of the manmialian ossicula auditus much more under- 

 standable. 



The most marked cliaracteristic o£ the Therapsid mandible 

 is that it is no longer hollow and box-like^ as in the Stego- 

 cephalia and the majority of the other reptiles, but that the 

 bones of the inside of each ramus are tightly pressed against 

 the outer bones, making it very narrow and solid. This 

 feature (which is not very easy to describe) is eminently 

 characteristic of the group and seems to occur in no other 

 reptiles. 



Pariasaiirus. 



The lower jaw of Pariasaurus has been somewhat incom- 

 pletely described by Seeley, but three jaws in the British 

 Museum enable one to get a clear idea of the principal lines 

 of its structure, although in one or two cases the sutures, 

 shown dotted in the tigures, are doubtful. The jaw is hollow, 

 all the bones except the articular simply surrounding a cavity. 



The dentaries meet in symphysis and pass backwards on 

 the outer side of the jaw to overlap the angular and surangular ; 

 it is doubtful if the dentary ever reaches the lower border of 

 the jaw — if it does so, it is only for a very small space just 

 in advance of the angular. The wide tooth-bearing area of 

 the dentary forms the roof of the cavity of the jaw. 



The splenial is a bone having a large symphysis with its 

 fellow of the opposite side ; it certainly forms a good deal of 

 the lower border of the jaw and apparently a good deal of 

 its outer surface, but here the suture is not certain. On the 

 inner aspect on each side tlie splenial is separated from the 

 dentary by a narrow slit-like vacuity, which is bounded out- 

 wardly by the upper margin of the splenial rising rapidly to 

 near the top of the jaw. Posteriorly the upper edge of the 

 bone is separated from the dentary by the anterior part of 

 coronoid, which is wedged in between the two bones. Tlie 

 hinder end of the splenial overlaps the prearticular and 

 angular, forming also a small part of the border of the inte- 

 rior mandibular vacuity. 



The coronoid is a small bone forming the front border of 

 the supra-Meckelian vacuity, and prolonged forward by a 

 thin strip which is wedged in between the splenial and 

 dentary. 



The surangular and angular are not separated by a satis- 

 factory suture, but are overlapped anteriorly by the dentary 

 and splenial, and form the outer and lower walls of the jaw ; 



