104 



BRITISH FOSSIL REPTILES. 



Fig. 14. 



Vertical longitudinal section of the cranium of a Crocodile 

 {Crocodilus acutus). 



ear ; from this plate extends a long and slender bony stem, which grows somewhat 

 cartilaginous, expands and bends down, as it approaches the tympanum or ear-drum, to 

 which it is attached. The cartilaginous capsule of the labyrinth or internal ear is 

 partially ossified by sinuous plates of bone connate with the neurapophyses (2 and 6), 

 between which that organ is lodged ; I apply the term ' petrosal' to the principal and 

 most independent of those ossifications of the ear-capsule, to that, e. g., which retains 



some mobility after it has con- 

 tracted a partial anchylosis to 

 the exoccipital (2), and which 

 appears upon the inner surface 

 of the cranial walls at the part 

 marked 16 in the subjoined 

 Cut 14, between 2 and 6. It 

 is the only independent bone 

 on that surface of the cranium 

 which, in my opinion, answers 

 to the ' petrous portion of the 

 temporal' in human anatomy, 

 and to which the term ' roclier' 

 can be properly applied, in the language of the French comparative anatomists. Cuvier, 

 however, restricts that name to the 'ahsphenoid' (6, figs. 13, 14) in the Crocodiles. 



The ossicles, (16 and 16'), together with the partial ossifications in the sclerotic 

 capsule of the organ of sight, (17, fig. 13) — always more distinct in Chelonia than in 

 Crocodilia — belong to that category of visceral bones to which the term ' splanchno- 

 skeleton' has been given ; they also are foreign to the true vertebrate system of the 

 skeleton. 



Thus the classification of the bones of the head of the Crocodiles, as of all other 

 vertebrate animals, is primarily into those of 

 The Endo-skeleton, 

 The Splanchno-skeleton, and 



The EXO-SKELETON. 



The bones of the endo-skeleton of the head form naturally four segments, called 



Occipital vertebra, N i, H i ; 



Parietal vertebra. Nil, H 11 ; 



Frontal vertebra, N iii, H iii ; 



Nasal vertebra, N iv, H iv. 

 These segments are subdivided into the neural arches, called 



Epencephahc arch (1 basioccipital, 2 exoccipital, 3 superoccipital, 4 connate 

 paroccipital) ; 



Mesencephalic arch (5 basisphenoid, 6 alisphenoid, 7 parietal, 8 mastoid) ; 



Fig. 13. 



