CROCODILIA. 



9 



form ' transverse processes ;' but such a vertebra, when analysed as it is developed, 



Kg. 7. 



diapophysis.^ 



-—-neural spine. 



parapophysis.' 



..neurapophysis. 



OJeEMTjggH Q l •»> >— pleurapophysi; 



"-hEemapophysis. 



■-• haemal spine. 



Ideal typical vertebra. 



resolves itself very nearly into the ideal type 

 given in the subjoined diagrammatic cut 

 (fig. 7) ; n is the neural axis, called ' myelon,' or 

 ' spinal marrow ;' h is the haemal axis, the chief 

 trunk of which is called ' aorta,' and ' caudal 

 artery.' The names of the vertebral elements 

 which, being usually developed from distinct 

 centres, are called ' autogenous,' are printed in 

 Roman type ; the Italics denote the ' exogenous' 

 parts, more properly called ' processes,' which 

 shoot out from the preceding elements. 



On comparing this form of the primary segment with that figured in Cut 4, 

 p. 5, Part I, it will be seen that they differ by altered proportions with some change of 

 position of certain elements ; but every modification resulting in the various forms of the 

 parts of the skeleton figured in T. XI, has its seat in one or other of the segmental 

 or ' vertebral' elements above defined ; and the same principle I believe that I have 

 established with regard to the internal skeleton in all vertebrate animals. 



With this preliminary explanation, the nature and relations to the typical vertebra of 

 the parts of the Crocodilian vertebrae, figured in T. IV, V, IX, will be, it is hoped, readily 

 appreciated. In T. IX, in which are figured, through opportunities kindly afforded 

 by the Marchioness of Hastings, and Searles Wood, Esq., F.G.S., some of the most 

 perfectly-preserved fossil reptilian vertebrae which have hitherto been discovered, the 

 elements and processes are indicated by the initial letter of their names. Figs. 1 and 2 

 give a side view and a back view of a cervical vertebra, apparently the fourth, of the 

 Grocodilus Hastingsia, from the Eocene deposits at Hordwell ; c is the centrum, n the 

 neural canal formed by the neurapophyses, which have coalesced superiorly with each 

 other, and with the neural spine (ns). Inferiorly they articulate by a suture (which is 

 shown by the wavy line on each side of the process d in fig. 1 ) with the centrum ; pi is 

 the pleurapophysis, which articulates by two parts, the lower one called the ' head' 

 to the process from the centrum, the upper one called the ' tubercle' to the process 

 from the neurapophysis ; beyond the union of the head and tubercle, the pleurapophysis 

 projects freely outwards and downwards, but instead of being elongated in that 

 direction, it becomes expanded in the direction of the axis of the body, i. e. forwards 

 and backwards, and so acquires a shape which has given rise to the name ' hatchet 

 bone' or ' hatchet-shaped process,' * applied to this element in the Plesiosanrus. 



* " To compensate for the weakness that would have attended this great elongation of the neck, the 

 Plesiosaurus had an addition of a series of hatchet-shaped processes on each side of the lower part of the 

 cervical vertebra." (Buckland, Bridgewater Treatise, vol. i, p. 206, and vol. ii, p. 30, 1836.) 



Cuvier recognised in these lateral bones, " en forme de hache," the homologues of the " petites cotes 

 eervicales" of the Crocodile. (Ossemens Fossiles, 4to, torn, v, pt. ii, p. 479, 1824.) And Conybeare had 



2 



