no BRITISH FOSSIL llEPTILES. 



The sacrum consists of two vertebrae onl}^ in Crocodilia as in Lacertilia : they are 

 modified in the present order, as before described, jj. 89. 



The skull consists, as we have also seen, of four segments. The hinder or occi- 

 pital surface of the skull presents, in the Crocodilia as in the Lacertilia, a single convex 

 occipital condyle, formed principally by the basioccipital, and not showing the trefoil 

 character which it bears in the Chelonia (PI. 11, fig. 4), in which the exoccipitals con- 

 tribute equal shares to its formation. In the Batrackia, the exoccipitals exclusively form 

 the joint with the atlas, and there are accordingly two condyles. The occipital region 

 of the crocodilian skull is remarkable for its solidity and complete ossification, and for 

 the great extent of the surface which descends below the condyle. (PI. 1 A, fig. 2.) 

 In the Lacertilia, a wide vacuity is left between the mastoid, exoccipital, and par- 

 occipital ; but in the Crocodilia this is reduced to the small depressions or foramina 

 near 3, fig. 2, PL 1 A. The tympanic pedicles (28) extend outwards and downwards, 

 firmly wedged between the paroccipital, mastoid, and squamosal ; in the Lacertians 

 these pedicles are suspended vertically from the point of union of the mastoid and 

 paroccipital. 



The chief foramen in the occipital region is that called 'foramen magnum' (between 

 2 and 2, in fig. 2), through which the nervous axis is continued from the skull. On 

 each side of the foramen magnum is a small hole, called ' precondyloid foramen,' for 

 the exit of the hypoglossal nerve. External to this is a larger foramen, marked n in 

 fig. 2, for the transmission of the nervus vagus and a vein. Below this is the 

 ' carotid foramen' c. All these are perforated in the exoccipital. Below the condyle 

 there is usually a foramen, and sometimes two, for the transmission of blood-vessels. 

 Lower down, at the suture between the basioccipital and basisphenoid, is a larger and 

 more constant median foramen, indicated by the dotted line from e t ; it is the bony 

 outlet of a median system of eustachian tubes, peculiar to the Crocodilia. On each 

 side of the median eustachian foramen, and in the same suture, is a smaller foramen, 

 which is the bony orifice of the ordinary lateral eustachian tube. The membranous 

 continuations of the lateral eustachian tubes unite with the shorter continuation from 

 the median tube, and all three terminate by a common valvular aperture, upon the 

 middle line of the faucial palate, behind the posterior or palatal nostril. The large, 

 bony aperture of this nostril is formed by the pterygoids (24 in fig. 2). The carotid 

 canal, c, opens by a short bony tube into the tympanic cavity, and is described as the 

 ' eustachian canal' in the ' Legons d' Anatomic comparce' of Cuvier. The artery 

 crosses the tympanic cavity, and enters a bony canal at its fore part, which conducts 

 to the ' sella turcica' in the interior of the cranium. 



The median eustachian foramen is described by Cuvier as the ' arterial foramen,'* 

 the canal from which divides and terminates in the ' sella turcica.'! By MM. Bronn, 



* Ossemens Fossiles, torn, v, pt. ii, p. 133. 

 t lb. p. 78. 



