136 ANATOMY OF VERTEBKATES. 



centrum; are concave mesially, fig. 94, 2, towards the brain-segment 

 which they protect, meeting above it to support the neural spine, 3 ; 

 they develope a petrosal plate, which meets a corresponding one 

 from the alisphenoid ; they give exit to the vagal and hypo- 

 glossal nerves, and send outward a strong jirocess, fig. 93, 4, 

 which articulates with the mastoid and tympanic. The anterior 

 and inner part of the base of this process is excavated by part 

 of the acoustic cavity : its outer extremity is rough for the attach- 

 ment of muscles : it thus repeats the essential characters of the ' par- 

 occipital ' in the Fish and Turtle ; but it is ossified, as an exogenous 

 transverse process, from the neurapophysis (exoccipital, 2). The 

 superoccipital, figs. 93 and 94, 3, is Inroad and flat, like the 

 similarly detached neural sj)ine of the atlas ; it advances a little 

 forward, beyond its sustaining neurapophyses, to protect the upper 

 surface of the cerebellum ; it is traversed by tympanic air-cells, 

 and assists with the ex- and par-occipitals, 2, 4, in the formation of 

 the ear-chamber. 



Proceeding with the neural arches of the Crocodile''s skull, if we 

 dislocate the segment in advance of the occiput, fig. 93, N" 2, we 

 bring away, in connection with the long base-ljone, 5, the bone, 9. 

 The two connate cranial centrums must be artificially divided, in 

 order to obtain the segments distinct to which they belong. The 

 hinder portion, 5, of the great base-bone, which is the centrum of 

 the parietal vertebra, is the basisphenoid. It supports that jiart 

 of the ' mesencephalon,' which is formed liy the lobe of the tliird 

 ventricle, and its upper surface is excavated for the pituitary 

 prolongation of that cavity. The basisphenoid developes from its 

 under surface a ' hypapophysis,' which is suturally united with the 

 fore part of that of the basioccipital,but extends further down, and is 

 similarly united in front to the ' pterygoids,' fig. 94, 24. These rough 

 sutural surfaces of the long descending process of the basisphenoid 

 are very characteristic of that centrum, when detached, in a fossil 

 state. The neurapophyses of the parietal vertebra, g, r, the ali- 

 sphenoids, protect the sides of the mesencephalon, and are notched 

 at tlieir anterior margin, for a conjugational foramen transmitting 

 the trigeminal nerve. As accessory functions they contribute, like 

 the corresponding bones in fishes, to the formation of the car- 

 chamber. They have, however, a little retrograded in position, 

 resting below in part upon the occipital centrum, and supporting 

 more of the spine of that segment, 3, than of tlieir own, 7. The 

 spine of the parietal vertebra (])arietal, figs. 93, 94, 95, r), is a single, 

 depressed bone, like that of the occipital vertebra ; it completes 

 the mesencephalic arch, as its crown or key-bone ; it is partially 



