Structure of the Plesiosaurus Macrocephalus. 631 



mities ; 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 con- 

 stant in its existence ; but the neurapophyses and their spines are the most 

 constant in regard to ossification : there is an obvious reason (in the import- 

 ance of the nervous chord which they are destined to protect), why these 

 parts should be firm when circumstances might forbid the consohdation of the 

 other vertebral elements. Thus, the neurapophyses are cartilaginous in the 

 Lampreys or Petromyzontidse, while the centrum is gelatinous, and they are the 

 firmest cartilaginous parts of the vertebras in the higher Chondropterygians. 



The neurapophyses and their spines are completely ossified in the Lepido- 

 siren, while the bodies of the vertebrae are represented by a fibro-gelatinous 

 chord, A similar condition appears to have obtained in the fossil Microdons 

 and some other osseous fishes, in which the ossified neurapophyses and hae- 

 mapophyses have been preserved, while the bodies of the vertebrae are lost. 



I now proceed to apply the preceding views of the elementary parts of a 

 vertebra to the exposition of the general characters of the vertebral column 

 in the Plesiosauri. 



Vertebral Column. 



The cervical vertebrae of the Plesiosauri generally present the following- 

 parts in a separate or unanchylosed state, — the centre, the neurapophyses, 

 and ribs ; and it is interesting to observe, that although, in general, no trans- 

 verse processes are developed in this region, an analogy with the structure 

 characteristic of this part of the spine in the Crocodile, is maintained in the 

 division of the articular surface for the cervical rib into an upper and lower 

 portion by a horizontal fissure ; which structure is well described and figured 

 by Mr. Conybeare in the Plesiosaurus Dolichodeirus. 



In Mammalia the interspace of the two cervical transverse processes is oc- 

 cupied by the vertebral artery : in birds by the vertebral artery and sympa- 

 thetic nerve : in the Plesiosaurus it is too inconsiderable to lead us to imagine 

 it to have been subservient to the protection of any important vessel or nerve ; 

 but its existence, besides being referable to the law of adherence to type, may 

 also have had relation to the presence of an interarticular ligament for the 

 purpose of firmly connecting the head of the cervical rib to the body. 



As the cervical vertebrae in the genus Plesiosaurus approach the dorsal, 

 the inferior part of the costal articulation becomes smaller, and a correspond- 

 ing increase of surface is afforded by the superior facet, which also gradually 

 rises from the centrum to the neurapophyses, and in the dorsal vertebraB stands 



