478 PROF. W. J. SOLLAS OX PLESIOSATJRUS 



axis, and since it has been polished reveals very clearly the arrange- 

 ment of the bones in this region of the sknll (fig. 14). 



Fig 14 — Transverse Section through the Skull of P. brachycephalus. 



(Scale j.) 



Pmac 



The upper and backwardly prolonged processes of the prsemaxillse 

 are seen on each side of the middle line above, the maxilhe bearing 

 teeth on each side, the vomers in the middle line below conjoined to 

 form a single trough-shaped bone ; a plate of bone forming the roof 

 of the mouth on each side of the vomers is an extension inwards of 

 the maxillse. 



The left orbit is complete and undistorted ; it is bounded by the 

 usual bones, the sutures between them being exceptionally plainly 

 shown. An additional bone, which looks as though it had been 

 segmented off from the jugal, appears, however, between the jugal 

 and postorbital ; its surface is marked in the same way as the jugal ; 

 and it bears a nutritive foramen, which completes the ascending 

 series of these openings carried by the jugal. The suture between 

 the two bones is distinct, however ; and the striations on their 

 surface are so directed as to indicate their separate nature. If a 

 posterior supraorbital bone had worked its way in between the jugal 

 and the postorbital, it would have the position here described ; but 

 simulation of the appearance of the jugal bone would remain unex- 

 plained. Hence it seems best to call it a suprajugal. 



The jugal and the suprajugal bones are not the simple bony plates 

 which they appear to be in a lateral view of the skull, and as the 

 jugal really is in recent Lacertilia. Both extend inwards behind the 

 orbit as a bony plate, which meets and joins externally with a 

 similar expansion of the parietal. In this way the orbit is com- 

 pletely walled in behind. The maxilla similarly extends inwards in 

 front, no doubt accompanied by the lacrymal ; and, below, a con- 

 tinuous floor is afforded by the expanded palatine. The orbit is 

 consequently very thoroughly walled round. 



The right orbit is incomplete ; and a horizontal fracture enables us 

 to remove the jugal, the only outer boundary bone of it remaining, 

 from the floor of the skull below. The jugal thus removed is a 



