52 



ANATOMY OF VERTEBRATES. 



45 



df 



Skeleton of Plesiosaurus. CLXIII. 



rises from the centrum to the 

 neurapophysis. This takes 

 place at the fortieth vertebra 

 in the Plesiosaurus homalo- 

 spondylus of the Whitby Lias, 

 but, in the PL doliclwdeirus, 

 fig. 45, of the Dorsetshire 

 Lias, at about the thirtieth, c. 

 The dorsal region is arbitrarily 

 commenced by the vertebra in 

 which the costal surface begins 

 to be supported on a diapo- 

 physis ; this progressively in- 

 creases in length in the second 

 and third dorsal, continues as 

 a transverse process to near 

 the end of the trunk, and on 

 the vertebra, s, between the 

 iliac bones, 62, it subsides to the 

 level of the neurapophysis. In 

 the caudal vertebras the costal 

 surface gradually descends from 

 the neurapophysis upon the 

 side of the centrum ; it is 

 never divided by the longitu- 

 dinal groove which, in most 

 Plesiosauri, indents that sur- 

 face in the cervical vertebras. 

 The neural arches are com- 

 monly 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 chiefly 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 

 outward and downward so as 

 to encompass the upper two- 



