526 Richard Owen, Esq., on the 



The number of vertebrae in the present species so characterised is 20 : that of the correspond- 

 ing vertebrae in the Plesiosaurus Harvkinsii, is 25. 



The vertebrae thus defined include, besides the ordinary dorsal, those which occupy the situa- 

 tion corresponding to the lumbar or ribless vertebrae in the crocodile ; but there are no vertebrae 

 of the trunk wanting ribs in the genus Plesiosaurus, in which structure it approximates the La- 

 certine Saurians. 



The special characteristic of the dorsal vertebrae of P^. Macrocephalus, as compared with PL 

 Harvkinsii, consists in their being (like the cervical) more flattened in the antero-posterior direc- 

 tion, and more concave at the sides ; in which latter particular they resemble the dorsal vertebrae 

 of the PL Dolichodeirus more than those of the PL Harvkinsii. 



At the commencement of the dorsal series of vertebrae in the present fossil, the bodies of the 

 vertebrae have been subject to a partial dislocation, and begin, as it were, to slide away from the 

 neurapophyses, until tlie dislocation becomes complete at the sixteenth vertebra : and the articular 

 depressions for the neurapophyses, and the canal for the spinal chord, are again brought into view. 



In the succeeding vertebrae the bodies gradually get into place again. Here the lower margins 

 of the neurapophyses begin to lose their angular form, and become rounded off, the articular de- 

 pressions presenting a corresponding figure. The transverse processes progressively increase in 

 length towards the middle of the trunk, and again diminish as they approach the tail : the bases 

 of the neurapophyses from which they rise diminish in vertical extent in the same ratio, and leave 

 a greater proportion of the centrum free from their embrace. The increasing length and upward 

 inclination of the transverse processes supporting the ribs, Mr. Conybeare has justly observed, 

 " seem intended to give a wider sweep to the ribs," and relates to the acquisition of greater ex- 

 pansion of the thoracic-abdominal cavity, where the largest viscera were lodged. 



The spinous processes at the beginning of the dorsal region diminish in antero-posterior extent, 

 but slightly increase in height : they then increase in both directions to the middle of the back, 

 and afterwards gradually decrease to the tail. 



Sacral Vertebrce. 



There are no sacral vertebras by anchylosis in the Plesiosauri. In the 

 present specimen, the dislocated state of the pelvic bones and corresponding- 

 region of the spine, renders it difficult to determine against which of the costal 

 processes of the vertebrge the extremities of the iliac bones may have abutted. 



In a specimen of the trunk of a Plesiosaurus Hawkinsii in the British Mu- 

 seum*, the two sacral vertebras are distinguishable by the sudden increase in 

 the breadth of their costal processes, as compared with those immediately pre- 

 cedinig and succeeding them. 



The short sacral ribs in the crocodile present this character in a still greater 

 degree, (as might be expected from its terrestrial habits); but we may per- 

 ceive in the Plesiosaur an evident additional affinity to the Emydo-Saurians, 

 as compared with the Ichthyosaur. The sacral vertebrae of the Plesiosaurus 

 Hawkinsii also differ from the lumbar and dorsal vertebrae preceding them by 



* See Plate XXV., Hawkins's Memoir on Ichthyosauri, &-c. 



