CROCODILIA. 93 



arch of the ' parietal vertebra,' as the segment is termed which is in advance of the 



occipital one. The hinder portion (5) of the great base-bone, which is the centrum of 



the parietal vertebra, is called 'basisphenoid.' Fig. 10. 



It supports that part of the ' mesencephalon,' 



which is formed by the lobe of the third 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' 



(24). These rough sutural surfaces of the long 



descending process of the basisphenoid are very Disarticulated mesencephalic arch, viewed 



characteristic of that centrum, when detached in fto"i bcliind (Crocodile). 



a fossil state. The neurapophyses of the parietal vertebra (G, 6) are called the ' ali- 

 sphenoids ;'* they protect the sides of the mesencephalon, and are notched at their 

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

 (see fig. 14, 6), resting below, in part upon the occipital centrum, and supporting more 

 of the spine of that segment (3) than of their own (7)- The spine of the parietal 

 vertebra (fig. 10, 7) is a permanently distinct, single, depressed bone, like that of the 

 occipital vertebra ; it is called the ' parietal,' and completes the neural arch, as its 

 crown or key-bone ; it is partially excavated by the tympanic air-cells. The bones 

 8, 8 wedged between 6 and 7, manifest more of their parapophysial character than 

 their homotypes (4, 4) do in the occipital segment, since they support modified ribs, 

 are developed from independent centres, and preserve their individuality. They form 

 no part of the inner walls of the cranium, but send outwards and backwards a strong 

 transverse process for muscular attachment. They afford a ligamentous attachment 

 to the haemal arch (fig. 13, H 11) of their own segment, and articulate largely with the 

 pleurapophysis (28) of the antecedent haemal arch (H iii), whose more backward dis- 

 placement, in comparison with its position in the fish's skull, is well illustrated in the 

 metamorphosis of the toad and frog. 



On removing the neural arch of the parietal vertebra, after the section of its 

 confluent centrum, the elements of the corresponding arch of the frontal vertebra, 

 slightly disarticulated, present the arrangement shown in fig. 11. The compressed 

 produced centrum (9) shown in natural connexion with the parietal centrum 5 in fig. 13, 



* This bone is the ' rocher' or petrous portion of the temporal bone, according to Cuvier, in the Reptiles 

 (Ossemens Fossiles, v, pt. ii, 1824) ; but is the 'aile temporalc du spheuoide' in fishes (Histoire Naturellc 

 des Poissons, torn, i, 1828), birds, and mammals. 



O 



