Skull of Mochlorhinus platyceps. 175 



tions of the pterygoid bones is 1/or inch. The palate begins 

 to contract in transverse width from the moment that the 

 pterygoid replaces the maxillary bone in forming its lateral 

 margin ; and behind the median post-narial vacuity, where 

 the convex contour of the anterior process is exchanged for a 

 contour concave from front to back, the transverse width of 

 the united pterygoid bones is IxV inch. The pterygoid bones 

 meet in the median line in a slight ridge; they are fractured 

 transversely just behind the bases of the slender processes 

 which appear to have been given off to the quadrate region, 

 and all the hinder part of the smooth post-palate, with the 

 adjacent bones, is lost. 



The superior surface of the pterygoids supports the remark- 

 able median plate first studied by Huxley in Ptychognathus 

 Marrayi. It extends upward as a vertical partition between 

 the orbits, and joins both the brain-case and its frontal prolon- 

 gation forward. This thin partition consists of several bones ; 

 the uppermost and most anterior is identified as the orbito- 

 sphenoid, and is most absolutely between the orbits, below 

 the front of the brain-case, and below the frontal bones. A 

 film of matrix may separate it from the bones below, which 

 may be compared with those of Dicynodon tigriceps figured in 

 the Phil. Trans. 18SS. On the posterior fracture this median 

 plate is shown as an extremely thin vertical film (fig. 1), but 

 matrix rests upon it supporting a vertical bone external to the 

 plate, which may be the element termed the columella or epi- 

 pterygoid, since it is between the parietal and pterygoid regions, 

 though neither its upward nor downward terminations are pre- 

 served. In front of it, manifestly rising from the pterygoid, are 

 the two films of bone which extend towards theorbito-sphenoid; 

 the hinder of these I have generally termed the median plate 

 of the pterygoid, but it now seems not improbable that this 

 thin ossification should be the alisphenoid, since it is imme- 

 diately under and apparently continuous with the brain-case 

 and extends downward to the sphenoidal region. An oblique 

 suture which extends upward and forward separates it from 

 the presphenoid. Both those bones appear to be truncated 

 above and to meet the orbito-sphenoid, though the matrix 

 already referred to either intervenes between the bones or 

 hides their junction. In front of the presphenoid there is 

 manifestly another bone immediately above the position of 

 the vomer. It is situate between the nares and extends 

 forward to the position of the nasal bones, so that it is in the 

 position of the ethmoid. 



All that remains of the occipital region is the median part 



