MancJicsier Memoirs, Vol. Ivii. (191 2), No. 1. 7 



The basis cranii is very similar in the two types, except 

 that in Ptei-oplax the carotid grooves, after curving round 

 the very large processi basipterygoidei, pass up the sides 

 of the bone to enter the pituitary fossa. 



The Exoccipital is a very small bone articulating 

 below with the basi-occipital and above with the opisthotic. 

 It does not appear to be pierced by any foramen. The 

 opisthotic and pro-otic are fused. Their joint upper 

 border has a distinct facet for the epiotic, which truncates 

 the grooves for the semicircular canals, which are freely 

 exposed on the iimer surface of the bone ; the opisthotic 

 also bears a short paroccipital process, which articulates 

 with a very distinct facet on the under side of the tabulare. 

 Between the pro-otic and the exoccipital is a triangular 

 notch, which no doubt gave exit to VII.? IX., X. nerves. 

 In front of the ear the inner surface of the pro-otic is 

 smooth, forming part of the brain case. The bone ends 

 anteriorly in a notched border, in front of which the other 

 cranial nerves seem to have passed out. In this region 

 the basisphenoid is excavated by the pituitary fo.ssa, the 

 sides of which are covered by a pair of processes which 

 rise as they pass forward to fuse with the parasphenoid 

 and ethmoid to form a thin vertical plate of bone, which 

 runs forward in contact with the roof of the skull, nearly 

 or quite to the premaxilla;. Reduction of this mass of 

 bone would easily give rise to the Sphenethmoid, a bone 

 which certainly occurs in some Stegocephalia (Myriodon). 



The palate of Pteroplax, though very different in 

 details from that of '' Loxoinnia," has a fundamental 

 resemblance that is very striking. The pterj'goids in the 

 two types are very similar, but in Pteroplax the articular 

 facet for the basisphenoid is carried on a distinct process, 

 and the bone appears to extend forwards to meet the 

 pre-maxilla. The sutures in this part of the skull are not. 



