78 STRUCTURE OF 'THE SKULL OF THE PYTHON. 



The basisphenoid, 5, and presphenoid, 9, form a single 

 bone, and the chief keel of the cranial superstructure. 

 The posterior articular surface looks obliquely upwards 

 and backwards, and supports that of the vertebral centrum 

 behind, as the posterior ball of the ordinary vertebra 

 supports the oblique cup of the succeeding vertebrae; 

 here, however, all motion is abrogated between the two 

 vertebrae, and the coadapted surfaces are rough and 

 sutural. The basisphenoid presents a smooth cerebral 

 channel above for the mesencephalon, in front of which a 

 deep depression (sella) sinks abruptly into the expanded 

 part of the bone, and there bifurcates, each fork forming 

 a short cul-de-sac in the substance of the bone. 



The alisphenoids, 6, form the anterior half of the fe- 

 nestra ovalis, which is completed by the exoccipitals ; and 

 in their two large perforations for the posterior divisions 

 of the fifth pair of nerves, as well as in their relative size 

 and position, the alisphenoids agree with those of the frog. 

 Each alisphenoid is a thick suboval piece, with a tuber- 

 cular process on its under and lateral part ; it rests upon 

 the basisphenoid and basioccipital, supports the posterior 

 part of the parietal and a portion of the mastoid, 8, and 

 unites anteriorly with the descending lateral plate of the 

 parietal bone. 



The parietal, 7, is a large and long, symmetrical, roof- 

 shaped bone, with a median longitudinal crest along its 

 upper surface, where the two originally distinct moieties 

 have coalesced. It is narrowest posteriorly, where it 

 overlaps the superoccipital, and is itself overlapped by 

 the mastoid : it is convex at its middle part on each side 

 of the sagittal spine, and is continued downwards and in- 

 wards, to rest immediately upon the basisphenoid. This 

 part of the parietal seems to be formed by an extension 



