92 BRITISH FOSSIL REPTILES. 



neck and beginning of the back, that of the occiput developes a ' hypapophysis,' but 

 this descending process is longer and larger, its base extending over the whole of the 

 under surface of the centrum. It is a character whereby the occipital centrum of a 

 Crocodilian reptile may be distinguished from that of a Lacertian one; for in the latter 

 a pair of diverging hypapophyses project from the under surface, as is shown in most 

 recent lizards and in the great extinct Mosasaurus* 



The upper and lateral parts of no. i present rough sutural surfaces, like those in 

 the centrums of the trunk, for articulating with the ' neurapophyses,' nos 2, 2, which 

 developc short, thick, obtuse, transverse processes (4, 4). The modified or specialized 

 character of the elements of the cranial vertebne has gained for them special names. 

 The centrum (1) is called the ' basioccipital ;' the neurapophyses (2,2) are the ' ex- 

 occipitals ;' the neural spine (3) is the ' superoccipital.' The transverse processes 

 (4, 4), which may combine both diapophyses and parapophyses, but which, from the 

 modifications of the transverse processes of the atlas, and the autogenous character of 

 the parapophyses in some fishes, and of the processes in question in the Chelonian 

 Reptiles, I believe to be best entitled to be regarded as the parapophyses, are called 

 the ' paroccipitals ;' they are never detached bones in the Crocodilia, as they are in 

 the Chelonia and in most fishes. 



The exoccipitals perform the usual functions of neurapophyses, and, like those of 

 the atlas, meet above the neural canal ; they are perforated to give exit to the vagal 

 and hypoglossal nerves, and protect the sides of the medulla oblongata and cerebellum 

 — the two divisions of the epencephalon. The superoccipital (3) is broad and flat, like 

 the similarly detached neural spine of the atlas ; it advances a little forwards, beyond 

 its sustaining neurapophyses, to protect the upper surface of the cerebellum : it is 

 traversed by tympanic air-cells, and assists with the exoccipitals (2, 2) in the formation 

 of the chamber for the internal ear. 



The chief modification of the occipital segment of the skull, as compared with that 

 of the osseous fish, or with the typical vertebra, is the absence of an attached htemal 

 arch. We shall afterwards see that this arch is present in the Crocodile, although 

 displaced ; a profile of it is given, as restored to its typical position, in the side view of 

 the bones of the skull, fig. 13. 



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

 segment in advance of the occiput, we bring away in connexion with the long 

 base-bone, 5 and 9, fig. 13, the bones which, in the same figure, are tied together by 

 the double lines, N 11, N in, and by the curved arrows, H 11 and H in. In fact, the 

 centrums of two vertebrae have here coalesced, as we find to happen in the neck of 

 the Siluroid fishes, and in the sacrum of birds and mammals. The two connate cranial 

 centrums must be artificially divided, in order to obtain the segments distinct to which 

 they belong. Fig. 10 gives a back view of the disarticulated bones of the neural 

 * See Quarterly Journal of the Geological Society, Nov. 1, )S49, p. 382, pi. x, figs. 5, 6. 



