970 MTi. w. P. piCRAFa' oif THE [Dec. 13, 



fossa it is continued forwards for a short distance in the form of 

 a vertically compressed lamina resting on the parasphenoid. 



The orhito- and prespheiwid are as yet represented only by 

 cartilage and are not to be separately defined. 



The ali^phenoid is more or less quadrate in shape. Its antero- 

 dorsal border runs along the orbital plate and postorbital region of 

 the frontal ; its postero-dorsal border is arched and \yedged in 

 between the parietal and squamosal (PI. LXI. fig. 3); its inferior 

 border is deeply hollowed to form the upper segment of the 

 trigeminal foramen ; its antero-interual border joins the still 

 membranous orbito-spheiioid, its lower angle conrributing to form 

 the optic foramen. 



The parasphenoid externally is perfectly distinguishable. It 

 may conveniently be divided into three regions : — (1) An elongated, 

 median rostrum ; (2) a pair of alisphenoidal wings ; and (3) a pair 

 of basitemporal wings, which last form the bnsitemporal plate 

 (PL LXI. fig. 3). In the ventral view of the skull of a young 

 Pi/goscelis papua from which this description of the parasphenoid 

 is taken, the anterior basicranial fontanelle and vestiges of basi- 

 pterygoid processes are plainly seen. The rostrum is continued 

 backwards to abut against the basisphenoid, expanding meanwhile 

 into a pair of wings to form the basitemporal plate. This plate 

 is narrow from before backwards, but extends laterally to the level 

 of the oater border of the mammillary processes. Its anterior 

 edge is free and forms the floor of the Eustachian grooves, which, 

 later, become converted into tubes (p. 962). Seen laterally, this 

 groove has the appearance of having been carved out of the base 

 of the skull so as to present a steep face, looking forwards and at 

 right angles to the main axis of the skull. The alisphenoid wing* 

 are separated from the basitemporal plate by a deep gorge, which 

 later becomes converted into a tube for the internal carotid artery 

 (PI. LXI. fig. 3). They overlap the suture between the ali- 

 sphenoid and prooric bones, and extend outwards as far as the 

 trigeminal foramen. Immediately above the carotid canal lies a 

 pneumatic foramen, which apparently terminates in the body of 

 the basisphenoid. In sagittal section the basisphenoid cannot 

 be distinguished from the basitemporal plate underlying it. 



The mesethmoid remains distinct for some time, in the form of 

 a vertical, linguiform plate of bone. Its posterior border is rounded 

 and imbedded in a large interorbital plate of cartilage. Its 

 anterior border is columnar. Its dorsal border expands laterally 

 under the frontals and nasals, and, eventually, turns downwards 

 as an ectoethmoidal ossification to rejoin the mesethmoid — forming 

 a large olfactory' cavity opening forwards and downwards into the 

 posterior region of the olfactory chamber. 



The squamosal varies somewhat in form. In Ajitenod^ytes, 

 CatarrJiactes, and Spheniseus it takes the form of a vertically 

 elongated bone. In the first-named its dorsal moiety is produced 

 into anterior and posterior limbs, giving the whole a Y-shaped 

 appearance ; of these two limbs the anterior is the more pronounced. 



