16 FOSSIL REPTILIA OF THE LONDON CLAY. 



The caudal hsemapophyses coalesce at their lower or distal ends, from which a 

 spinous process is prolonged downwards and backwards ; this grows shorter towards 

 the end of the tail, but is compressed and somewhat expanded antero-posteriorly. 

 The haemal arch so constituted has received the name of ' chevron bone.' 



A side view of the body of a middle caudal vertebra of the Crocodiks toliapicus- is 

 given in T. Ill, fig. 8, and an under view of the same in fig. 9, showing the two 

 hypapophysial ridges extending from the articular facets for the hsemapophyses at 

 one end to the other end of the centrum. 



The segments of the endo-skeleton composing the skull are more modified than 

 those of the pelvis ; but just as the vertebral pattern is best preserved in the neural 

 arches of the pelvis, which are called collectively ' sacrum,' so, also, is it in the same 

 arches of the skull, which are called collectively ' cranium.' The elements of which these 

 cranial arches are composed preserve, moreover, their primitive or normal individuality 

 more completely than in any of the vertebras of the trunk, except the atlas, and 

 consequently the archetypal character can be more completely demonstrated. 



In fossil Crocodilia, and many other reptiles, the bones of the head are very liable 

 from this cause to a greater extent of dislocation and separation than happens to the 

 skull of the warm-blooded animal, in which a greater proportion of those primitive 

 bones coalesce with age. It not unfrequently happens that detached bones of the 

 skull of a reptile are found fossil, and the usually much modified form of these vertebral 

 elements renders their determination difficult. In order to diminish this difficulty, 

 especially as the bones of the cranium are least familiar to the palaeontologist in their 

 detached state, I have subjoined a side view of them, fig. 9, nearly as they are arranged 

 in the formation of the successive natural segments of the skull. Such figures are the 

 more necessary in the present state of anatomy and palaeontology, since the illustrations 

 of the osteology of the crocodile which have hitherto been prefixed to the descriptions 

 of the fossil remains of the Reptilian class, as, e. g., in the great work of Cuvier, 

 include only figures of the bones in question as they are naturally combined together in 

 the entire skull. 



For the anatomical description and determination of the individual bones, as con- 

 stituent elements of the vertebral segments of the head, I must refer the reader to my 

 work 'On the Archetype of the Vertebrate Skeleton,' pp. 115-25, figs. 18-21, and 

 pi. 2, fig. 3 ; and here limit myself to an exemplification of the natural arrangement 

 and names of the bones according to the letters and numbers in figure 9. 



The bones of the head of the Crocodiles, as of all other vertebrate animals, are 

 primarily classified into those of 

 The Endo-skeleton, 

 The Splanchno-skeleton, and 

 The Exo-skeleton. 



