460 



BRITISH FOSSIL REPTILES. 



deposits. The subject of the present section, as gigantic as the Maestricht 

 Mosasaur, manifests an extreme modification of the Plesiosauroid type of structure. 

 It is probable that the large Pterodactyles of the same geological period, soaring 

 like albatrosses and giant petrels over the Cretaceous seas, co-operated with the 

 marine reptiles, as those sea birds now do with cetaceous mammals, in reducing 

 the excessive numbers of the teeming tribes of fishes, and in maintaining the 

 balance of oceanic life. 



SUPPLEMENT NO. I. 

 CRETACEOUS LIZARDS. 



Tribe — Natantia. 



The genus Leiodon was defined (p. 195) on a modification of Mosasauroid teeth 

 in a fragment of jaw discovered in a Cretaceous formation in Norfolk.' 



The vertebral column of Leiodon anceps (Cut, fig. 1) exhibits the range of modifi- 

 cation in its several regions which I deem to be characteristic of the great extinct 



Fig 1. 



"^^^mm^ 



Eestoeation and Type-veetebe^, Leiodon anceps. 



' Lacertia natantia,' and offers a strong contrast with the comparative uniformity 

 of the vertebras in Pi/tJion, PalaopJiis and other Ophidia (pp. 135 — 154). 



The atlas (fig. 2) consists of a pair of neurapophyses, n, and a detached 

 hsemapophysis, h, simulating a centrum : of this the transverse exceeds the vertical 



1 See 'Report of the British Association' for 1841, 8vo., p. 1-44. 



