84 BRITISH FOSSIL REPTILES. 



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

 Yig. 7. resolves itself very nearly into the ideal type 



t. — neural spine. givcu lu tlic subjoincd diagrammatic cut 



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

 ^n%- •••'•■"""'""''°''''^'''' ' spinal marrow ;' // is the haemal axis, the chief 



^"''"'''"'""'•'-■..^-.fr^^r^p-., trunk of which is called ' aorta,' and ' caudal 



cziio^Tia>MJoES>-i''™"p<'p''y=>s- artery.' The names of the vertebral elements 



parapcphys,s.----'' ^\^V^ ^^"'^^' ^^^^^ usually dcveloped from distinct 



XV/y ■■•--ha^inapophyB.s. ceutrcs, are called ' autogenous,' are printed in 



zvgapopi.piu.--''' W '^ Roman type ; the italics denote the ' exogenous' 



W"'--- hiemai spine. parts, uiore properly called 'processes,' which 



Ideal typical Tcvtcbra. shoot out from the preceding elements. 



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

 p. 5, 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 PI. 1, 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 Plates 1 D, 3, will be, it is hoped, 

 readily appi'eciated. In Plate 1 D, in which are figured 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 CrocodUits Ilasdiif/nia:, 

 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 ) vdt\\ 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 Plesiosaurus. 



* " 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 vertehra." (Buckland, Bridgcwatcr Treatise, vol. i, p. 206, and vol. ii, p. 30, 1836.) 



Cuvier recognised in these lateral bones, " cu forme de liache," the homologues of the " petites cotes 

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



