106 SKULL OF THE CROCODILE. 



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, resting below in part upon the occipital cen- 

 trum, and supporting more of tl^e spine of that segment, 

 3, than of their own, 7. The spine of the parietal verte- 

 bra 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, and 

 overlaps the superoccipital. The bones, 8, 8, wedged 

 between 6 and 7, manifest more of their diapophysial 

 character than their homotypes, 4, 4, do in the occipital 

 segment, since they support modified ribs, are developed 

 from independent centres, and preserve their individu- 

 ality. They form no part of the inner walls of the cra- 

 nium, but send outwards and backwards a strong trans- 

 verse process for muscular attachment. They aflbrd a 

 ligamentous attachment to the haomal arch of their own 

 segment, and articulate largely with the pleurapophyses, 

 28, of the antecedent haemal arch, whose more backward 

 displacement, 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 present 

 the same arrangement. The compressed produced cen- 

 trum has its form modified like that of the vertebral cen- 

 trums at the opposite extreme of the body in many 

 birds ; it is called the " presphenoid." The neurapophy- 

 ses, 10, 10, articulate with the upper part of 9 ; they are 



