CRETACEOUS FORMATIONS. 7 



more commonly it is bent, as it is in the tooth of the great Pterodactyle from the 

 Chalk figured in the above-cited ' Monograph on Cretaceous ReptiHa,' Tab. XXVIII, 

 fig. 5. In general, the transverse section of the crown is less truly elliptical than that of 

 the base, owing to its being a little flattened on one side. The smaller teeth, probably 

 from the back part of the dental series, are rather more curved than the larger ones 

 (Tab. Ill, figs. 16—20). 



VertebrcB of Fterodacti/hs. Tab. I, figs. 11 — 14. Tab. II. 



The most instructive specimens from the Cambridge Green-sand are those which 

 have afforded the precise and hitherto unknown characters of certain vertebra3 of 

 Pterodactylus. Viewed as indicative of the generic character of these bones, they 

 give the earliest known example of the " procoelian" type of vertebrse* in the Reptilian 

 class, being the first cup-and-ball vertebrae, with the "cup" at the fore part of the 

 centrum, met with in the ascending order of strata. It cannot be doubted that this 

 structure prevails in the moveable vertebrse of the neck and back of all Fterosauria, 

 and must be predicated of the Dimorphodonf of the Lias as well as of the Pterodactylus 

 of the Green-sand, in which the structure is now clearly demonstrated. The chief 

 difference which the Pterodactyle presents in this respect from modern Lizards is, 

 that both the cup and ball are of a more transversely extended elliptical shape in 

 most of the vertebree of the flying Saurian. 



Amongst the numerous vertebrae submitted to me were specimens of united, or 

 partly united, " atlas and axis." 



The atlas consists of a centrum (Tab. I, figs. 11 and 12, c), of two slender 

 styliform neurapophyses (ib.^ n), and of a very small discoid neural spine. The centrum 

 is so short as to be discoid ; it is flat w^here it joins and becomes anchylosed to the 

 axis {x) , and is concave for the occipital tubercle : this cup is circular ; its depth is 

 shown in the section of the anchylosed atlas and axis, fig. 12. The neurapophyses (w), 

 resting on each side of the upper half of the centrum of the atlas, converge and articu- 

 late above with two small tubercles, as shown in fig. 13, on the fore part of the neural arch 

 of the axis ; the neurapophyses almost meet, but do not unite above the neural canal. 



The body of the axis is eight times longer than that of the atlas ; it expands 

 posteriorly, and terminates by a transversely elliptical ball (h) at the upper part of that 

 end, and in a pair of thick, short, obtuse, diverging apophyses (p), at the lower part. 

 There is a rudimental hypapophysial ridge, fig. 1 2, h, from the middle and toward the 

 fore part of the under surface of the centrum ; the extent to which this surface 



* 'Monograph of Fossil Reptilia of the London Clay,' 4to, vol. for 1850, p. 11. 

 f ' Reports of the British Association,' 1858. 



