LIASSIC DINOSAURS. 101 



trum (PI. 48, fig 1, c), the liypapopliysis (ib., hy), and the neurapopliyses (ib. n, «), 

 of the atlas. 



The centrum of the atlas is small, and has been anchylosed with that of the axis 

 (PI. 48, fig. 2, x). Its vertical section is subquadrate, longitudinally grooved on 

 each side; broader above than below. The hypapophysis (fig. 1, hy) is broader, 

 but less deep, than the centrum, and the bases of the neurapophyses (ib., n,n,) 

 extend down the sides of the centrum to articulate with the hypapophysis. 



The neurapophyses (ib., n,n) are ununited above, as they are below, and have 

 yielded to the oblique pressure ; but the slight dislocation seems to have taken place 

 without fracture of an upper union. There is no trace of spinous process ; but above 

 the neural arch of the atlas is a pair of large, thick, transversely oblong, dermal 

 bones or scutes ; the fi'actured surface of the most entire one, to the right (ib., r), 

 is 2 inches in length by 1 in depth ; it exposes a compact peripheral texture of 

 4 or 5 lines in thickness at the upper and outer part, and about 1 line at the under 

 or inner part, with a fine cancellous structure between. To the broad hypapo- 

 physis of the atlas was articulated a long and slender rib {pleurapophijsis) (ib., 



In the foregoing constitution of the vertebral segment succeeding the skull we 

 have the reptilian condition of the atlas, with modifications most closely repeated 

 by the Crocodilia amongst the existing members of the class. The CrocodiUa alone 

 show the transverse extension of the hypapophysis or "pseudo-centrum " of the seg'- 

 ment, associated with the presence of articulations for the pleurapophysial elements. 

 In lizards free pleurapophyses are not developed from the atlas or from the axis, 

 rarely from the third cervical vertebra. But in Scelidosaurus the atlantal hypapo- 

 physis is relatively broader than in CrocodiUa, and there is no trace of the detached 

 representative of the neural spine which characterises the atlas in Crocouiha.^ 

 In Plesiosaurus and Ichthyosaurus the true centrum of the atlas progressively 

 acquires its form and proportions as such, and in the same degree resembles, in 

 its relations to the basi-occipital, and to its own neural arch, the centrum of the 

 first trunk-vertebra in fishes. The hypapophysis is proportionally reduced in size, 

 and forms the first of the " sub-vertebral wedge-bones " in the Ichthyosaums? 



The second block of Lias (PL 48, fig. 2) includes the bodies of the axis {x) and 

 of the third cervical vertebra, with parts of the pleurapophyses of the atlas 

 {pi. a) and axis {pl.x) ; it includes, also, large, massive, dermal bones external and 

 superior to the vertebral elements. The centrum of the axis is 1 inch 3 lines in 

 length, from the line of adhesion of that of the atlas, part of which remains 

 connected as the " odontoid process ;" the proper body of the axis is subcarinate 



1 'Annals and Magazine of Natural History,' vol. xx, pp. 217 — 225. 

 ^ Ichthi/opterygia, pi. xix, fig. 5, liy, a; pi. xsiii, fig. 2. 



