1899.] OSTEOLOGY OF THE PTGOPODES. 1029 



Tbe exocdpital. — Externally it is bounded supero-internally by 

 the supraoccipital, and supero-esternally by that portion of the 

 prootic cartilage which lodges the floccular fossa. Its internal 

 border is < -shaped. The posterior limb is free and bounds tbe 

 foramen magnum, the anterior runs along the outer border of the 

 basioccipital. Its external border has fused with the prootic 

 cartilage, the boundary between the two being indicated by a 

 notch superiorly. The vagus foramen pierces it near its centre. 

 Internally, it is largely concealed by the opisthotic, only its inner 

 half being nsible. The vagus foramen appears on this side at the 

 base of the opisthotic, and indicates how much of the exoccipital 

 is concealed by this bone. 



The supraoccipital is cleft in the middle line superiorly for more 

 than half its length. Its ventri-lateral border is fused with the 

 epiotie, but the distinction between the two bones can yet be made 

 out. Its internal does not differ much from its external form. 



The prootic appears externally in the form of an oblong mass of 

 cartilage separating the squamosal from the anchylosed opisthotic 

 and exoccipital bones. Internally its size is seen to be considerable. 

 Supero-dorsally it is notched to form the inferior border of the 

 floccular fossa. Its outer half hes immediately in front of the 

 squamosal, which bone it almost entirely shuts out from the inner 

 surface of the cranium, only a small semicircular strip of about 

 I mm. being visible, and forming the floor of a groove separating 

 the prootic from the parietal and alisphenoid bones. Its antero- 

 ventral border is linguiform and imbedded in a mass of cartilage. 

 Its inner lateral border is in part (superiorly) fused with the 

 opisthotic and in part(inferiorly) imbedded in cartilage in common 

 with the linguiform anterior end. The meatus internus is very 

 deep. 



The epiotie is only ossified at its junction with the supraoccipital ; 

 the rest of the posterior vertical semicircular canal is yet carti- 

 laginous, and forms the superior boundary of the floccular fossa. 



The opisthotic has almost completely fused with the prootic, but 

 traces of the original suture still remain. It fuses posteriorly 

 with the exoccipital. The vagus foramen passes between its 

 antero-intemal border and the exoccipital in front. There is no 

 trace of the opisthotic visible externally. 



The hasisplienoid is not visible externally, being underfloored by 

 the parasphenoidal rostrum and its basitemporal plate, with which 

 furthermore it has now completely fused. Below the pituitary 

 fossa, and that portion of the basisphenoid immediately behind it, 

 is a large air-sinus, and this forms the only indication of the 

 division between the para- and basisphenoidal regions. The 

 pituitary fossa forms a moderately deep pit. The dorsum sellse is 

 yet membranous. In the adult the pituitary fossa seems to have 

 become almost obliterated (p. 1025V Immediately behind the 

 pituitary fossa the basisphenoid is marked by a deep < -shaped 

 notch, dividing a median portion from a pair of lateral wings. 

 The membrane stretched between the two wings forms the dorsum 



