BRITISH FOSSIL REPTILES. 49 



The costal processes or ribs are considered by Geoffroy St. 

 Hilaire* to undergo in the Cetacea a similar change of direction, 

 and also a dislocation from their usual attachments, and to have 

 their distal extremities bent downwards and anchylosed to a 

 rudimental spine, so as to assume the form and perform the 

 offices of chevron-bones, or hsemapophyses ; but as the horizon- 

 tal processes of the caudal vertebrae in the Cetacea (as exempli- 

 fied in the skeleton of a young Balaam untarctica, fourteen 

 feet in length, which I have lately had the opportunity of ex- 

 amining,) are originally developed from distinct centres, and in 

 distinct cartilages, they appear to me to represent, with the 

 corresponding separate vertebral elements in the Plesiosaurs, 

 the true costal appendages of the tail, and the hsemapophyses 

 must therefore be regarded as other and different elements of 

 the vertebra. This view is supported by the fact that the long 

 transverse processes supporting the ribs in the thoracic region of 

 the spine in the same young whale, have no osseous nuclei de- 

 veloped in them, but are continuous cartilages from the still 

 unossified parts of the centrum. I may observe also that the 

 haemapophyses in the young Cetaceans examined by me, exhibit 

 what appear to be their permanent condition in the Enaliosau- 

 rians, viz. a want of bony union at their distal extremities ; at 

 least I have never yet observed a true chevron-shaped bone, 

 such as results from the anchylosis alluded to, in any skeleton 

 of an Enaliosaurian. 



Of the vertebral elements above enumerated the centrum is 

 the most constant in its existence, but the neurapophyses and 

 their spines are the most constant in regard to ossification : and 

 there is an obvious reason, in the importance of the nervous 

 cord which they are destined to protect, why these parts should 

 be firm and resisting when circumstances might forbid the con- 

 solidation of the other vertebral elements. Thus, the neurapo- 

 physes are cartilaginous in the Lampreys, or P ctromy zontida; y 

 while the centrum is gelatinous. The neurapophyses and their 

 spines are completely ossified in the Lepidosiren,while the bodies 

 of the vertebrae are represented by a fibro-gelatinous cord. A 

 similar condition appears to have obtained in the fossil Micro- 

 donts and some other osseous fishes, in which the ossified neur- 

 apophyses and hasmapophyses have been preserved, while no 

 trace of the bodies of the vertebrae remains. 



CHARACTERS OK THE GENUS PLESIOSAURUS. 



I now proceed to apply the foregoing views of the elementary 

 parts of a vertebra, in the first place, to the exposition of the 

 * Mi'm. dir Museum, ix. p. 1 1.?. 



VOL. VIII. 1839. K 



