• a Pachypodfroin the Cambridge Ui}])er Qreensand. 317 



■ridge has become depressed, and the side is rounded, so tliat 

 the flattened articular end has an aspect of being a little com- 

 pressed from side to side. 



These vertebn^, if really belonging to the same individual 

 as the foot-bones, would indicate a smaller and more mammal- 

 like tail than that attributed to the other Dinosaurs. Judging 

 from Prof. Owen's figures (Palteont. 1862), the early caudal 

 vertebra of Scelidosaurus have the centrum more obliquely 

 inclined forward, a neural arch with a longer attachment, 

 longer and stronger transverse processes placed more anteriorly, 

 and an absence of ridges on the side of the centrum, which has 

 the articular margin more thickened ; but the absence of ridges 

 from the centrum is the most marked character of Scelido- 

 sauruSj which distinguishes its caudal vertebrae from those of 

 this animal. 



The caudal vertebrse of Hylxeosaurus have not been figured 

 by Prof. Owen. 



The caudal vertebra3 of Hypsilopliodon, so far as can be 

 judged from Prof. Owen's figure (Palteont. 1854, pi. 1), ajjpear 

 to be not dissimilar, but have the transverse processes from 

 the centrum more developed and placed anteriorly instead of 

 posteriorly, while the articular margins of the centrum seem 

 to be greatly developed. In Iguanodon (Paleeont. 1854, pi. 9, 

 and 1851, pi. 37) the resemblance to the centrum oi Acaatlio- 

 pkolis is much closer (supposing the figured determinations to 

 be satisfactory), and the differences would seem to be chiefly 

 in the proportions of the bones. Presuming that most of the 

 Dinosaurian caudal vertebrae from the Potton Sands are to be 

 referred to Iguanodon^ it will be noticed that the centrum is 

 more elongated than in AcantJiophoUs, and has but one ridge 

 on the middle of the side of the centrum, while the basal sur- 

 face is not so concave from front to back, nor the parts of the 

 side so concave or convex respectively from above downward. 



In Hadrosaurus the centrum, as figured by Leidy, appears 

 to be much shorter from back to front, and not likely to be 

 confounded with Acanthoptliolis. 



On comparing the fossil with reptiles, the cup-and-ball 

 articulation, the long attachment of the neural arch, and the 

 strong transverse processes (not to mention the number of 

 vertebra) show the tail of lizards to be well distinguished 

 from Acanthopholis. In Chelydra {Emysaura)^ where the 

 Chelonian tail is long and has the vertebras in some respects 

 comparable, the centrum is opisthocoelian. 



Among crocodiles the articular ends of the centrum are 

 flattened instead of being concave, and the centrum differs in 

 most of its details ; but of all reptiles the crocodile is least 

 unlike this Dinosaur, though no crocodilian vertebra have the 



