CRETACEOUS ENALIOSAURS. 459 



a large proportion 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 ^, y ; the 

 upper contour has been contributed by the neurapophyses, the articular surfaces 

 for which may be discerned at n, />, on each side of the smooth neural tract, "> 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 vertebrae 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 FolyptychodoH apply as to the posterior 

 cervical vertebrae (PI. 31, figs. 1 and 2) of more ordinary plesiosaurian proportions. 

 Between that vertebras and the axis I infer, therefore, that the anterior cervicals 

 rapidly diminished in length, and that the anterior ones exhibited the same 

 Ichthyosaurian shortness as they do in Pliosaurus. The magnitude of the head, 

 jaws, and teeth, o{ Polypti/cliodoii 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 

 (p. 220), the phalanges of which are figured in ' Enaliosauria,' PI. 30, belonged to 

 FoJypiijchodon. 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 (p. 183), But whilst Mosasaurus, by its vertebral, palatal, and dental 

 characters, foreshadows the saurian type to follow, Polyptychodon adheres more 

 closely to the prevailing type of the sea-lizards of the great geological epoch then 

 drawing to its close. 



The seas in which the English Chalk hills and cliffs were formed, and by which 

 they were modified in the course of upheaval, must have teemed with life, and 

 have been traversed by shoals of fishes needed for the sustentation of the numerous 

 kinds of large marine reptiles now known to have existed during that period, and 

 all of which were provided with jaws and teeth adapted, under diverse secondary 

 modifications, to the capture and destruction of the finny races. Of these carni- 

 vorous reptiles some, as e.g. Ichthyosaurus campylodon (p. 223) and Plesiosaurus Bernardi 

 (214), were large species of genera represented throughout the oolitic period ; 

 others, as e.y. Leiodon (p. 195) and Mosasaurus (p. 183), ofi"er generic or family 

 modifications of the Saurian structure, unknown in any other than the Cretaceous 



