SAUROPTERYGIA. 245 



with the inferior interspace between the bodies of the atlas 

 and axis. As the cervical vertebrae approach the dorsal, the 

 lower part of the costal pit becomes smaller, the npper part 

 larger, until it forms the whole surface, gradually rising from 

 the centrum to the neurapophysis (fig. 93). This takes place 

 at the fortieth vertebra in the Plesiosaurus homalosrpondylus of 

 the Whitby Lias, but, in most species, at about the thirtieth. 



The dorsal region is arbitrarily commenced by the vertebra 

 in which the costal surface begins to be supported on a di- 

 apophysis : this progressively increases in length in the 

 second and third dorsal, continues as a transverse process to 

 near the end of the trunk, and on the vertebra between the 

 iliac bones it subsides to the level of the neurapophysis. In 

 the caudal vertebra the costal surface gradually descends from 

 the neurapophysis upon the side of the centrum ; it is never 

 divided by the longitudinal groove which, in most Plesiosauri, 

 indents that surface in the cervical vertebrae. The neural 

 arches are commonly unanchylosed with the centrum. The 

 long and large spinous processes, in contact along the trunk 

 and base of the neck,* must have restricted the bending 

 movements to the lateral directions. The pleurapophyses 

 gain in length, and lose in terminal breadth, in the hinder 

 cervicals ; and become long and slender ribs in the dorsal 

 region, curving outwards and downwards so as to encom- 

 pass the upper two-thirds of the thoracic abdominal cavity. 

 They decrease in length and curvature as they approach 

 the tail, where they are reduced to short straight pieces, 

 as in the neck, but are not terminally expanded ; they 

 cease to be developed near the end of the tail. The haema- 

 pophyses in the abdominal region are subdivided, and with 

 the haemal spine or median piece, form a kind of " plastron " 

 of transversely extended, slightly bent, median and lateral, 

 overlapping bony bars, occupying the subabdominal space 



* See the fine example of PL homalosponclylus in the British Museum. 



