24 FOSSIL REPTILIA OF THE 



Atlas and Axis (Tab. VI). 



The centrums of the first and second cervical vertebra coalesced, as in 

 Plesiosaurus, from the same locality and formation as the hinder cervical vertebra, 

 Tab. V, present the proportions, in regard to their antero-posterior diameter, of 

 the cervical vertebrae of Pliosaurus ; but they belong, in all probability, to 

 the same Plesiosauroid reptile as the vertebrae previously described, and I refer 

 them to the genus Polyptychodon. 



Like most of the fossils from the Haslingfield locality, they have been subject 

 to attrition. The contour of the centrum of the atlas (fig. 1) has been subcir- 

 cular ; its anterior articular surface ( c , a,) is concave, and has afforded a large pro- 

 portion of the bottom or middle part of the cup for the occipital condyle. The 

 lower part of the cup has been completed, as in Plesiosaurus, by a wedge-shaped 

 hypapophysis, the articular surface for which is shown at h, y\ the upper contour 

 has been contributed by the neurapophyses, the articular surfaces for which may 

 be discerned at %, p, on each side of the smooth neural tract n , in figs. 2 and 3. 



The line of the original separation of the bodies of the atlas and axis may be 

 traced ; the second hypapophysis, or part of it, remains anchylosed to their inferior 

 interspace ; it has been much smaller than the first. The posterior surface of the 

 centrum of the axis vertebra (fig. 2, c, x) is almost flat, showing the Plesiosauroid 

 nature of the bones. In the similarly short vertebra; of an Ichthyosaurus, this 

 surface would have been deeply concave. 



Having thus a proof of the plesiosauroid nature of these anchylosed vertebrae, 

 the same grounds for referring them to Polyptychodon apply, as to the pos- 

 terior cervical vertebra (Tab. V, figs. 1 and 2) of more ordinary plesiosaurian 

 proportions. Between that vertebra and the axis I infer, therefore, that the 

 anterior cervicals rapidly diminished in length, and that the anterior ones exhi- 

 bited the same Ichthyosaurian shortness as they do in Pliosaurus. The mag- 

 nitude of the head, jaws, and teeth, of Polyptychodon resembled that of its 

 more ancient congener from the Kimmeridge Clay, and the supporting part of the 

 spinal column appears to have been shortened and strengthened accordingly. 



It is probable that the large Plesiosauroid paddle, from the Chalk of Kent, the 

 phalanges of which are figured in the 'Monograph on the Fossil Reptilia of the 

 Cretaceous Formations,' for 1S51 (Palaeont. Soc), pi. 17, belonged to Poly- 

 ptychodon. Thus the evidence at present obtained respecting the huge but hitherto 

 problematical carnivorous Saurian of the Cretaceous period proves it to have 

 been a marine one — the rival and contemporary of the equally huge Maestricht 

 lizard. But whilst Mosasaurus, by its vertebral, palatal, and dental characters, 

 oreshadows the saurian type to follow 7 , Polyptychodon adheres more closely 



