PROCOLOPHON FEOM THE CAPE COLONY. 803 



tical bone, which extends inward and forward so as to form tti9 

 back wall of the orbit, but does not visibly enter into the wall of 

 the brain-case. This bone is evidently a downward fcH of the 

 parietal, and in meeting the quadrate offers a modification which is 

 paralleled to some extent among Lizards and Ehynchocephalians. 

 The quadrate bone is also developed outward into a large, vertical, 

 cellular mass, which extends outward for T 3 ^ inch beyond the arti- 

 culation for the lower jaw, is behind the quadrato-jugal, and in- 

 creases the transverse width of the skull to 2 T 2 ^- inches, which is 

 considerably more than its length. The pterygoid process of the 

 quadrate is thin, is directed inward, upward, and forward, and is 

 given off at more than T 2 7 inch above the articular surface of the 

 quadrate bone ; it may extend inward for about the same distance. 

 The pterygoid bones meet in the middle line, and are prolonged 

 backward and obliquely downward to a level with the articular 

 pedicles of the quadrate bones ; they are not in contact with the 

 base of the brain- case. 



The vertical fracture through the brain-case is somewhat oblique. 

 This cavity appears to have been T 4 F inch deep in the sphenoidal 

 region, and slightly narrower ; and, as in most reptiles, it is like a 

 box contained within the skull *. The basisphenoid is thick and 

 channelled on its ventral aspect, two strong compressed hypapophyses 

 being prolonged downward and backward towards the pterygoid 

 bones. The alisphenoid is thick and directed upward and outward ; 

 there is a large perforation placed between it and the bone above ; 

 and, since the perforation opens into the back of the orbit, it pro- 

 bably gave passage to the optic nerve, so that the bone would be 

 the frontal. It is uncertain whether the brain-case was Closed in 

 above by bone, no bone being visible in the upper median part of 

 the cerebral region. The fracture in the specimen gives no evidence 

 concerning the existence of a columella. 



The Systematic Position of Procolophon. 



Such being the more important structures of the skull of the genus 

 Procolophon, it remains to determine the place of the type in the 

 Eeptilian series. Professor Owen placed the genus in the family 

 Mononarialia of his order Theriodontia, the family being defined as 

 having " the external nostril single or undivided," and with more 

 than three incisor teeth in each premaxillary bone. The more per- 

 fect preservation of the nasal region in the specimens described, by 



* The specimen figured by Professor Owen in his South- African Reptile 

 Catalogue, pi. xx. fig. 2. as the brain-cavity of Nythosaurus larvatus may be 

 seen, by comparison with the lateral view of the same specimen (pi. xx. fig. 1), 

 to represent the whole interior cavity of the skull from the brain forward to 

 the nares. There is no evidence that the cavity for the cerebrum extended 

 forward between the orbits, as indicated in the figures and text (p. 24). This 

 apparently mammalian character would also result from a modification of the 

 Chelonian plan of a median prolongation forward of the chamber for the envelope 

 of the brain. 



3n2 



