52 



ANATOMY OF VERTEBRATES. 



45 



RlcoU'liiii of ricslosaums. OLXIII. 



rises from the centrum to the 

 neurapophysis. This takes 



i'place at the fortieth vertebra 

 in the Plesiosaurus liomalo- 

 spontlylus of the Whitby Lias, 

 but, in the PI. dolichodeirus, 

 fio-. 45, of the Dorsetsliire 

 Lias, at about the thirtietli, 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 vertebrre the costal 

 surface gradually descends from 

 the neurapophysis upon the 

 side of the centrum ; it is 

 never divided by the longitu- 

 dinal groove wliich, in most 

 Ph'siosanri, indents that sur- 

 face in the cervical vertebra\ 

 The neural arches are com- 

 monly unanchylosed with the 

 centrum. The lono- and lara;e 

 spinous processes, in contact 

 along the trunk and base of the 

 neck, nuist have restricted the 

 bending movements chiefly to 

 the lateral directions. The 

 plcurapophyses gain in length, 

 and lose in terminal breadth, 

 in the hinder ecrvicals ; and 

 become long and slender ribs 

 m the dorsal region, curving 

 outward and downward so as 

 to encompass the upper two- 



