Skull of MochloYhinus platyceps. 171 



animals by blending with the parietal, just as the inter- 

 parietal appears to blend commonly with the supraoccipital. 

 When it is present, the preparietal has the aspect of being the 

 key-structure of the roof of the brain-case, lying in front of 

 the parietal foramen. 



These three bones form the hinder part of the roof of the 

 skull above the brain-case j they meet the frontal bones 

 anteriorly. 



Each frontal is an elongated oblong bone, and the pair 

 unite by a slightly undulating median suture which extends 

 forward just over the rounded angle between the face and the 

 upper surface of the skull ; their extremities converge in front 

 and diverge as they extend backward. 



The length of the median frontal suture is about 2 inches, 

 though their extremities extend back so as to make each bone 

 3^ inches long. The transverse width over the frontal bones 

 in the middle of the orbits is about 2y'o inches, and, owing to 

 the elevation of the margin of the orbit, the superior surface 

 is transversely concave. 



The transverse width of the skull increases in front owinff 

 to the way in which the prefrontal bones, which make the 

 anterior corners of the orbits, are ])rolonged outward and 

 downward, giving the orbits a position (fig. 1) which is entirely 

 lateral, where the eyes look outward and very slightly 

 upward. The prefrontal bone is only well preserved on the 

 left side. 



The postfrontal bone joins the frontal at the back of the 

 orbit by a suture which is easily traced, and that bone thus 

 enters into the upper surface of the skull, joins the parietal 

 bone behind, and extends backward upon that bone so as to 

 form the inclined concave borders of the temporal vacuities. 

 This backward extension of a film of the postfrontal upon 

 the parietal is substantially the same condition as is seen in 

 the corresponding region in Cynognathiis and other Therio- 

 donts, notwithstanding the circumstance that in that group of 

 animals the parietal bones form a median knife-like edge, and in 

 this and many other Dicynodonts those bones form a broad flat- 

 tened crown between these plates of the postfrontal. The 

 suture is not usually seen, because it runs in the line of the 

 slightly elevated ridge which defines the temporal concavity. 

 Between this suture and the suture which limits the frontal 

 bone there is manifestly another suture on the upper surface 

 of the skull, defining a long narrow oblique bone which enters 

 into the orbit at its upper hinder angle. It is about 1^ inch 

 long and about y^ inch wide on the orbital border. It appears, 

 theretbre, to be in the position of the postorbital bones; for 



