6G Geological Society. 



costal processes. The transverse and oblique processes are instances 

 of the second, or exogenous parts of a vertebra. 



The vertebrae of the Plesiosaurus are then described according to the 

 preceding views, and the varying relations of the different vertebral 

 elements in different regions of the spine are pointed out. 



The dorsal vertebras having been determined in previous descrip- 

 tions of the skeletons of this genus by their usual anatomical charac- 

 ter of affording articular surfaces to ribs, much difficulty has been ex- 

 perienced in denning the precise number of the cervical vertebrae, in 

 consequence of the gradual change of the cervical ribs (hitherto con- 

 sidered as transverse processes), from the form of an expanded hatchet 

 to that of an elongated style. The author, however, regarding the 

 lateral appendages of the spinal column throughout its whole extent 

 as modifications of one and the same vertebral element, proposes to 

 distinguish the cervical and dorsal regions of the spine by the position 

 of the articular surface supporting that lateral element, or rib : thus he 

 would call cervical, all those anterior vertebras in which the body af- 

 fords the whole or any part of the costal articular surface ; and the 

 dorsal series would commence at that vertebra where the costal sur- 

 face had first passed upon the neurapophysis. The author finds in 

 the Plesiosaurus Hawkinsii that the costal processes of the two ver- 

 tebras which are articulated to the ilium, and which are consequently 

 to be regarded as sacral, begin again to slide down from the neurapo- 

 physis upon the centrum ; and that in the PL macrocephalus, where 

 the costal appendages are lost, the bodies of the first two vertebras 

 which again begin to exhibit a portion of the costal pit, correspond, 

 in their relative situation to the ilia, with the sacral vertebras in the 

 more perfect skeletons of the PL Hawkinsii. In the vertebras which 

 succeed the sacral ones, the ribs rapidly descend from the neurapo- 

 physes upon the centrum ; but the bodies of the caudal vertebras so 

 characterized "may be distinguished from those of the cervical by the 

 absence of a longitudinal groove which traverses the costal pits in 

 the cervical region; and.'also by the presence of the articular surfaces 

 for the hasmapophyses. The determination of characters in the body 

 or central element of a vertebra which point out the region of the 

 spine to which it belongs, is the more valuable in the skeletons of 

 the Enaliosauri, because in these cold-blooded reptiles ossification is 

 tardy in its progress, and anchylosis of the autogenous elements of a 

 vertebra rarely takes place ; and hence the bodies are often found 

 separated and detached from their peripheral appendages. 



After concluding his observations on the structure of the vertebras 

 in the Plesiosauri generally, the author next proceeds to point out 



