SAUROPTERYGIA 225 



The dorsal region is arbitrarily commenced by this vertebra, 

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

 apophysis, which 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 above or 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 vertebras. The neural 

 arches remain long unanchylosed with the centrum in all 

 Plesiosauri, and appear to be always distinct in some species. 

 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 

 encompass 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 hsemapophyses in 

 the abdominal region are subdivided, and with the haemal 

 spine or median piece, form a kind of " plastron " of trans- 

 versely-extended, slightly bent, median and lateral, overlapping, 

 bony bars, occupying the subabdominal space between the 

 coracoids and pubicals. In the tail the hsemapophyses are 

 short and straight, and remain re-united both with the centrum 

 above and with each other below. The haemal spine is not 

 developed in this region. This modification has been expressed 

 by the statement that there were no chevron-bones in the 

 Plesiosaur. The tail is much shorter in the Plcsio- than in 

 the Ichthyosaurus. 



The skull is depressed ; its length is rather more than 

 thrice its breadth; but the proportions somewhat vary in 

 different species. The cranial part, or that behind the orbits, 



Q 



