PLIOSAUEUS. 



15 



clown to the posteroexternal angle. The two or three hindmost cervical ribs are 

 longer and more slender than those in front and form a transition to the dorsal type. 

 Their articulation is single, or at least the separation between the head and tubercle is 

 only very slightly marked and does not amount to a real division. Cervical ribs in this 

 genus have been figured by Lydekker, Quart. Journ. Geol. Soc. vol. xlvi. (1890) pi. v. 

 fig. 2 (see also text-fig. 4). 



The dorsal ribs are very strongly developed ; they articulate by a single head, the 

 concave surface of which is oval, the long axis being vertical. Immediately external 

 to the head the rib narrows rather quickly for a short distance, then remains about the 



Cervical ribs of left side of Pliosaurus ferox. The articular end and the anterior face of each are shown 

 A, from the anterior part of the neck ; B and C, farther back. (R. 2446, | nat. size.) 



ft., head of rib ; p., posterior angular process ; t., tubercle of rib. 



same thickness to its lower end. It is somewhat compressed from before backwards and 

 was probably grooved slightly along its posterior and perhaps its anterior surface, but 

 owing to the crushing undergone by the specimens this is not certain. Some dorsal ribs 

 of a very young individual (R. 3537) have been found in a quite uncrushed condition 

 and present peculiar features. The articular head is small, its diameter bein<* 

 almost the same as that of the rib just outside it ; its facet is flat and nearly circular 

 with a slight ventral angle. External to the head, the rib runs first straight, then 

 curves down, thickening gradually till near its distal end, when it again contracts and 



