50 REPORT— 1839. 



generic characters of the vertebral column in the Plesio- 

 sauri. 



The most conspicuous and remarkable feature of this part 

 of the skeleton, is the extraordinary length of its cervical portion, 

 which includes from twenty to forty vertebrae. The articular 

 surfaces of the bodies of the vertebrae are either flat or slightly 

 concave, or most frequently convex in the centre and concave 

 at the periphery. In general the bodies present two pits at 

 their under part, but this character is not constant. 



The cervical vertebrae of the Plesiosauri generally present the 

 following parts in a separate or unanchylosed state, — the 

 centrum, the neurapophyses, and ribs ; and it is interesting to 

 observe that although, in general, no transverse processes are 

 developed in this region, an analogy with the structure charac- 

 teristic 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 on each side is occupied by the vertebral artery : in 

 Birds, by the vertebral artery and sympathetic 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 referrible to the law of 

 adherence to type, may also have had relation to tiie presence 

 of an interarticular ligament for the purpose of connecting the 

 head of the cervical rib or hatchet-bone 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 corresponding increase of surface is afforded by 

 the superior facet, which also gradually rises from the centrum 

 to the neurapophyses, and in the dorsal vertebrae stands boldly 

 out as a true transverse process from the upper side of the base 

 of each neurapophysis. 



At the sacral vertebras, however, the transverse processes 

 subside to the level of the neurapophyses ; and as the caudal 

 vertebrae recede from the trunk, the articular surface, which, as 

 in the neck, represents, or is in the situation of, the transverse 

 process, gradually descends, and passes from the neurapophysis 

 to the side of the centrum ; but it is not divided by the longitu- 

 dinal groove which characterizes the costal surface in the neck. 



This groove is more marked in some than in other species of 

 Plesiosaurus ; and I have seen Plesiosaurian vertebrae undoubt- 

 edly cervical, in which no trace of it was visible. 



The neurapophyses are commonly unanchylosed with the ver- 



