Geological Society. 67 



the specific peculiarities of the cervical, dorsal, sacral, and caudal 

 vertebrae of the Plesiosaurus macrocephalus . 



The bodies of the cervical vertebrce of this species may be distin- 

 guished from those of the PL Hawkinsii and PI. dolichodeirus by the 

 close proximity of the costal to the neurapophyseal depressions : in this 

 respect, indeed, the anterior cervical vertebrae of the PI. mctcrocephalus 

 diiFer from those of every Plesiosaurus which the author has examined. 

 Other minor distinctive characters are also pointed out. The number 

 of cervical vertebrae in the PI. macrocephalus is twenty-nine, that of 

 PI. Hawkinsii thirty-one ; the length of the neck is twice that of the 

 head ; in PL Hawkinsii it is three times the length of the head. 



The dorsal vertebrae of the PL macrocephalus differ from those of 

 the PL Hawkinsii and PL dolichodeirus in being more flattened in the 

 antero-posterior direction, and more concave at the sides ; true trans- 

 verse processes are developed from the neurapophyses to support the 

 ribs, as in other Plesiosauri. 



In the sacral vertebrae the medullary canal presents a slight en- 

 largement as compared with that in the neck. 



The terminal caudal vertebrae in the specimen described are wanting, 

 but in those of a perfect skeleton of the PL Hawkinsii in the British 

 Museum, the author discovered an interesting modification of the sur- 

 faces by which the bodies are joined to one another. They are hol- 

 lowed out like the vertebrae of the Ichthyosaurus, so as to join by 

 double concave surfaces; he conceives this to be, as in the Batrachian 

 reptiles, the original structure of all the vertebrae, and that it is per- 

 manent in those which are most remotely situated from the centre 

 , and source of vital energy : but Mr. Owen observes, that this arrest 

 of development is obviously designed, to give to the tail of the Ple- 

 siosaurus the same combination of elasticity with flexibility, which 

 characterizes that of fishes. 



After describing the vertebral and sternal ribs of the abdominal 

 region, the author next compares the bones of the pectoral and pelvic 

 extremities with those of other species of Plesiosauri. In the macro- 

 cephalus the ulna is relatively longer and broader, and presents a 

 more complete reniform figure than in the Hawkinsii or dolichodei- 

 rus. These characters are still more marked in the fibula ; the femur 

 is longer than the humerus. There are eight ossicles in the carpus, 

 and six ossicles in the tarsus ; these latter are so arranged as to allow 

 of greater freedom of inflection forwards, and to give a compound 

 motion to the stroke of the hinder paddle. 



The author concludes with a detailed account of the structure of 

 the cranium, which he compares, at each step, with that of the two 



F 2 



