18 FOSSIL REPTILIA OF THE LONDON CLAY. 



Hyoidean arch (39 epihyal, 40 ceratohyal, 41 basihyal) ; 



Scapular arch (50 suprascapula, 51 scapula, 52 coracoid) and appendages 

 (53 — 58 bones of fore-limb). 

 The bones of the splanchno-sheleton, are 



The petrosal (16) and otosteals (16') ; 



The sclerotals (17) which in most retain their primitive histological condition 

 as fibrous membrane. 



The turbinals (is and 19) and teeth. 

 The bones of the exo-sJceleton, are 



The lacrymals (73). 



The superorbitals (present in Alligator sclerops). 



To the foregoing brief analysis of the constituent parts of the framework of the 

 Crocodilia, which are petrifiable or conservable in a fossil state, and from the study 

 and comparison of which we have to gain our insight into the nature and affinities of 

 the extinct Reptiles, it seems here only requisite to add a few observations on the 

 characteristic mode in which the bones are associated together in certain parts of the 

 skeleton in the present order, and especially in the skull. 



With regard to the trunk, the Crocodilia are distinguished from the Lacertilia and 

 from all other existing orders of Reptiles, by the articulation of the vertebral ribs 

 (pleurapophyses) in the cervical and anterior part of the dorsal segments by a head 

 and tubercle to a parapophysis and diapophysis. As this double joint is associated 

 with a doable ventricle of the heart, and as the single articulation of every rib in other 

 Reptiles is associated with a single ventricle of the heart, we may infer a like difference 

 in the structure of the central organ of circulation in the extinct reptiles, manifesting 

 the above-defined modifications in the proximal joints of the ribs. 



The sacrum consists of two vertebrae only, in Crocodilia as in Lacertilia : they are 

 modified in the present order, as before described, p. 14. 



The skull consists, as above defined, of four segments. The hinder or occipital 

 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 (Part I, T. XV, 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. (T. VI, 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, T. VI. The tympanic pedicles (28) extend outwards and downwards, 

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



