OEGANIZATION OF THE OENITHOSAUEIA. 89 



ward and forward over the cerebral lobes towards tbe portion of 

 tbe frontal bone which formed the hinder wall of the orbit. The 

 antero-posterior extent of this cerebral mass (as preserved ; for 

 it is somewhat fractured behind) is ^\ inch ; but on the under 

 side of the fossil (PL XI. fig. 3, c m), where this brain-region is well 

 defined between the bones on each side of it (fig. S,s), it is y^g-inch 

 wide, and extends forward for -j^ inch, thus demonstrating that 

 while it is seen for -^ inch behind the cerebrum, it also extends 

 forward under the cerebrum for -^ inch. Therefore I identify 

 this cerebral mass as the cerebellum, and infer from the antero- 

 posterior convexity of its exposed superior portion that its hinder 

 outline was vertical, and did not extend much behind the part of 

 the brain preserved. The ridge over the cerebrum is due to a 

 blood-vessel. 



The resemblance of form and arrangement of parts between this 

 fossil animal's brain and the brain of a bird (PI. XI. fig. 1) amounts, 

 as far as the evidence goes, to absolute identity. This is manifest 

 on comparing a cast of the brain-cavity of a bird with the natural 

 mould of the brain-cavity of the Cambridge G-reensand Pterodac- 

 tyle. The cerebrum being the cerebrum of a bird, the optic lobes 

 those of a bird, and the cerebellum that of a bird, no more perfect 

 specimen could add to the force of the conclusion that the 

 Ornithosaurian brain is an avian brain of typical structure. 

 It seems to me, therefore, an inevitable conclusion that the Orni- 

 thosauria are members of the same great class as birds, and are 

 separated from carinate and other birds only by such charac- 

 ters as divide mammalian or reptilian orders of animals from 

 each other — that is to say, by modification of the skeleton. If 

 this claim to admit the Ornithosauria, on account of vital struc- 

 tures, into the class Aves is allowed, then it follows that the 

 skeletal modifications of Ornithosaurians are as much avian 

 structures as the skeletal modifications of the Cetacea, Carni- 

 vora, and Monotremata are all mammalian structures. 



Turning to the skeleton in Ornithosaurian animals, I propose to 

 point out the characters of the several bones without regard to 

 theoretical conception of the Ornithosaurian organization. On 

 a priori grounds it would be reasonable to expect that no greater 

 variations from a common avian plan would be presented than are 

 seen in the variations from the mammalian common plan shown 

 by Edentates, bats, and whales, or are presented by the variations 

 of the several orders of reptiles from the common plan of the Rep- 

 tilia. I do not think it will be found that the variations from the 



