ON A NEW SPECIES OF PLESIOSAURUS 533 



minate above by a smooth and rounded edge. The antero-lateral 

 margin of these portions of the atlas is nearly straight, the upper 

 third or thereabouts being inclined at a great angle to the rest : the 

 postero-lateral margin is greatly excavated, on account of the back- 

 ward projection of the supero-lateral pieces above and of the cornua of 

 the inferior piece below. The body of the axis is concave posteriorly, 

 the sides are convex from above downwards. Below it is rather con- • 

 ■cave from behind forwards. Its posterior margin is straight ; the 

 anterior is also straight, except for a short distance inferiorly, where 

 it is much bevelled off Traces of a rib, which probably articulated 

 with the OS odontoideum, exist on both sides of the anterior part of 

 the body of the axis. 



Between its anterior edge and the posterior excavated margin of 

 the parts of the atlas just described, there is an interspace of ^th of 

 an inch. This is filled by a mass of bone with a convex edge, and 

 separated by a deep groove from the axis and the rest of the atlas. 

 Superiorly this bony mass is overlapped by the supero-lateral piece of 

 the atlas — inferiorly by one of the cornua of the inferior piece ; but 

 ■on cutting this cornu away, I found it rested on a sort of articular face 

 furnished by the inferior continuation of the bony mass. But this 

 passed below, without any visible line of demarcation, into the second 

 ■" subvertebral " bone. This bone is convex below and in front (where 

 it fits into the excavated margin of the inferior piece of the atlas), 

 and behind slopes backwards to articulate with the bevelled face of 

 the axis. 



I have nowhere seen the structure of the anterior articular cup, 

 and the sutures which unite the supero-lateral and inferior pieces of 

 the atlas, displayed as they are in this specimen ; ^ but many of the 

 other peculiarities are as well shown in one or other of the specimens 

 of P. Hawkinsii in the British Museum. 



Thus the under surfaces of the atlas and axis are exhibited in the 

 specimen I have called No. i. They are a good deal broken away, 

 so as to display a longitudinal and nearly horizontal section of these 

 vertebrae. The axis has nearly the same form and size as in P 

 Etheridgii ; the inferior piece of the atlas appears to be bent upwards, 

 and to be broken inferiorly and posteriorly ; but between the two is 

 seen a thin bony disk, not more than a third as long as the axis, and 

 whch I take to be the section of the bony plate-like mass interposed 

 between the axis and the three anterior pieces of the atlas in P. 

 Etheridgii. No. 2 shows the right side of the axis and atlas very 



' I may observe, that I performed all the more delicate operations required in bringing 

 •out these parts myself. 



