170 



Prof. H. G-. Seeley. On the Classification [Nov. 24, 



the development of the pneumatic condition is sufficiently general 

 among Sauropoda and Theropoda, to show that these groups are 

 united together by a character which separates them from Stegosauria 

 and Ornithopoda. It is not possible to form an opinion as to the 

 inference which should be drawn from this character concerning the 

 vital organisation of the animals in which it is found. For, many 

 of the armoured genera have the neural arch much extended verti- 

 cally, in the dorsal region, showing that the lungs were greatly 

 developed. But since the difference in height between the carapaces 

 of flat-shelled Emydian Chelonians and Tortoises, is chiefly due to 

 differences in the volume of the lungs, it is quite possible that 

 considerable variations in osteologncal character may occur in the 

 vertebrae, without much difference in the vital organ which produces 

 the change. On the other hand it must be remembered that among 

 existing animals, the pneumatic skeleton is only found iu birds. 



Of late years the Dinosaurian skull has become well known. 

 Mr. J. W. Hulke, F.R.S., described the brain-case of Iguanodon in 1871 

 (' Geol. Soc. Quart. Journ.,' vol. 27, p. 199), and in 1874 I described the 

 base of a cranium (' Greol. Soc. Quart. Journ.,' vol. 30, p. 690) which 

 was named Craterosaurus Pottonensis. In the former the brain-case is 

 closed in front, and the basi-sphenoid has a comparatively slight 

 downward development, while in the latter the base of the skull is 

 much more like that of Hatteria than it is like Iguanodon. These 

 types include so far as the evidence goes all the forms of skull hitherto 

 discovered. On the plan of Iguanodon are shaped the skulls of 

 Hypsilophodon and apparently Diclonius, while the skulls of 

 Dipiodocus and Ceratosaurus have much in common with Cratero- 

 saurus in having the deep pituitary depression, the anterior part of 

 the brain-case open, &c. The evidence concerning the skull is very 

 imperfectly known, but, so far as it goes, points in the same direction 

 as the other characters in indicating that there are probably only two 

 types in the group. Any classification must necessarily be provisional 

 until the skulls and skeletons which exist are adequately described. 

 The considerations adduced appear, however, to show that the 

 Dinosauria has no existence as a natural group of animals, but includes 

 two distinct types of animal structure with technical characters in 

 common, which show their descent from a common ancestry rather 

 than their close affinity. These two orders of animals may be 

 conveniently named the Ornithischia* and the Saurischia, and defined 

 by the following characters. 



OrnitJiischia. 



In this order the ventral border of the pubic bone is divided, so that 

 one limb is directed backward parallel to the ischium as among birds, 

 * " Ischia " is used bj Aristotle for the pelvis. 



