BRITISH FOSSIL REPTILES. 47 



separate cartilages, and exogenous, or those which shoot out as 

 continuations from these independent constituents. The auto- 

 genous, or true elements, are, 



1st. The centrum, or body of the vertebra, which, in Mam- 

 malia, as Cuvier has observed, is complicated by two epiphyses. 



2nd. Two superior laminae developed to protect the great 

 nervous cord which rests on the upper surface of the centrum, 

 and which I have therefore proposed to call neurapojjiiyses*. 



3rd. Two inferior laminae developed, generally to protect the 

 great blood-vessels on the under surface of the centrum, and 

 which I have proposed to call hcemapophyses \. 



4th. The superior process :|: which is connected and generally 

 anchylosed with the distal extremities of the neurapophyses, 

 and forms, in conjunction with those processes, the superior 

 arch of the vertebra. 



5th. An inferior spinous process, which is connected, and 

 commonly anchylosed with, the distal extremities of the hmma- 

 pophyseSy forming, in conjunction with these, a chevron or 

 V-shaped bone. 



To the category of autogenous vertebral pieces belong the ribs, 

 which generally are anchylosed to the other vertebral elements 

 in the cervical, sacral, and caudal vertebrae of the warm-blooded 

 vertebrate classes. 



The propriety of regarding the ribs as vertebral elements is 

 well illustrated, in the Plesiosaurus, in the cervical, sacral, and 

 caudal vertebrae of which they have been generally described 

 as transverse processes, although they are separate bones. 



These elements bear the same relation to the centrum and its 

 true transverse processes which the spinous processes do to the 



* They are \\\e periaux, oi perivertebral elements, of GeofFroy St. Hilaire. 



f These are the clievrou-hones of Mr. Conybeare, the paraanx or paraverte- 

 bral elements, of GeofFroy St. Hilaire ; terms which he also applies to the costal 

 processes, regarding these in the abdominal and thoracic regions as the ex- 

 panded halves of the chevron-bones. If I had adopted GeofFroy 's term, ' paraal,' 

 or its English equivalent, ' paravertebral element,' I must have diverted it from 

 its original sense, in which it is applicable to two distinct elements, viz. the ribs 

 and chevron-bones, which will be seen to co-exist in certain vertebrae of the 

 Enaliosauri, and some existing animals ; and I have preferred, therefore, to 

 invent and define a new term, which has the advantage of expressing a physio- 

 logical relation ; and I am happy in being able to cite the authority of Cuvier 

 for the propriety of this step. Flis words are, in reference to an analogous 

 case, " Donner a un mot connu im sens nouveau est toujours un precede dan- 

 gei'eux, et, si Ton avait besoin d'exprimer une idee nouvelle, il voudrait mieux 

 inventer un nouveau terme, que d'en detourner ainsi un ancien." Mem. du 

 Mus., tome ix. p. 1 23. 



X This is regarded by GeofFroy, but without due grounds, to consist essen- 

 tially of two lateral moieties, termed, epiaux ov epii^ertcbral tiementB. 



