82 BRITISH FOSSIL REPTILES. 



retain throughout hfe that distinctness, or moveable union with the rest of their 

 segment^ which they manifest at their first appearance ; and they are then classified as 

 distinct bones from the rest of their segment, to which the term 'vertebra' is 

 restricted, and which is equally regarded as a single bone ; as, e. g., in the dorsal 

 region of the skeleton. In the cervical region the whole segment is called ' vertebra,' 

 and is recognised as the equivalent bone to a dorsal vertebra, although it includes the 

 costal elements, because these have coalesced with the rest of their segment, which 

 anchylosis is misinterpreted as a mere modification of a transverse process ; and the 

 ' cervical vertebra' is distinguished by having that process ' perforated,' and not 

 entire as in the other vertebrae. 



But, in the Crocodile, the embryonic condition of the cervical ribs in Man is 

 retained throughout life ; and, therefore, if we were to be guided by the characters laid 

 down by the recognised authorities in anthropotomy for the classification of its vertebrae, 

 we should seek in vain for any vertebrae with " transverse processes perforated for the 

 transmission of vertebral arteries," whilst we should find all the vertebrae from the 

 head to the loins, "with articular surfaces, either on their sides or their transverse 

 processes, where they join with ribs," and should accordingly have to reckon these as 

 " dorsal vertebrae." 



These and many similar instances which might be adduced, have compelled me to 

 premise a few brief explanations of the principles and nomenclature by which I shall 

 describe the fossil remains of the JRejjlilia, and illustrate their nature by reference to the 

 skeletons of their existing representatives, in the present Work. 



The primary segment of the skeleton of all Vertebrata is a natural group of bones, 

 which may be severally recognised and defined under all the modifications to which 

 such segment may have been subjected in subservient adaptation to the habits and 

 exigencies of a particular species. 



A view of such a segment, as it exists in the thorax of the crocodile, the tortoise, 

 and the bird, is given at p. 5. 



The part marked c is the ' centrum,' or body of the vertebral segment ; it is 

 always developed originally as a separate element, and retains its character of 

 individuality in the tortoise and crocodile. The bony arch above the centrum was 

 formed originally by two distinct side-plates, — the ' neurapophyses,' «, which coalesce 

 with one another at their summits and thence develope a median plate or process of 

 bone called the ' neural spine' ns. Other bony processes which shoot out from the 

 neurapophyses are more variable, and will be afterwards noticed. The arch so formed 

 coalesces with the centrum in the bird, and constitutes an apparently single bone, to 

 which, in anthropotomy, the name ' vertebra' would be restricted. But it would be 

 as reasonable to confine it to the central element (c) in the tortoise and crocodile ; for 

 the parts of the inferior arch are not less essentially parts of the same natural segment, 

 than the neurapophyses which have formed the upper arch. The next pair of elements, 



