LOWER LIAS. 3 



The modifications of the spinal column of the trunk and tail of the Scelidosaur 

 can thus be studied and compared in sixty consecutive vertebrae of one and the 

 same individual. 



Cervical Vertebra. Tab. I. 



The fracture of the matrix including the skull has passed through the centrum 

 (Tab. I, fig. 1, c ), hypapophysis (\h.,hy), and neurapophyses (ib. n , n), of the atlas. 



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

 axis (Tab. I, fig. 2, «■). The vertical section of it is subquadrate, longitudinally 

 grooved on each side ; broader above than below. The hypapophysis (fig. 1, Ay) 

 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 fractured 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 hypapophysis 

 of the atlas was articulated a long and slender rib (pleurapophysis) (ib., fig. 2, pi, a ). 



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 Crocodilia 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 Crocodilia, and there is no trace of the detached 

 representative of the neural spine which characterises the atlas in Crocodilia.* 

 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 Ichthyosaurus.* 



The second block of Lias (Tab. I, fig. 2) includes the bodies of the axis ( x ) 



* See descriptions and figures of the modifications of the atlas and axis in my paper on the 

 homologies of those bones in the ' Annals and Magazine of Natural History,' vol. xx, pp. 217 — 225. 



