24 Mr. H. G. Sccley on the Structure of 



as the fossil is broken behind, this comparison does not de- 

 termine exactly the resemblance and difference between the 

 bones. The lateral orbital spaces are larger and better-marked 

 in the bird, and similarly approximate mesially ; but while in 

 the Inrd the eyes abut against the front of the brain, in the 

 Omithosaur they are removed further forward, and conse- 

 quently the triangular space which comes between and behind 

 the orbits in Oniithosaurians is in front of them in birds ; and 

 in these animals the bone which I have previously named the 

 ethmoid bone (the orbito-sphenoid of Prof. Owen) is of such 

 shape as would fit on to that space. Finally, the frontal of the 

 bird is largely excavated behind to cover the cerebrum. From 

 tlie divergence of the excavations in the Ornithosaur frontal, it 

 is clear that they are not for the cerebrum, but for the olfactory 

 lobes in front of the cerebrum, which lobes, when developed, 

 are commonly divided. And if any one will compare the 

 figure of this bone here given with fig. 3, pi. 11 of my book on 

 the Ornithosauria, where is shown the suture of the parietal 

 bones from which the frontals have come away, it will be 

 evident that a considerable piece is wanting from the back of 

 this frontal bone, which, like the bird's frontal bone, is thereby 

 proved, when perfect, to have partly, if not largely, covered 

 the cerebnim. Here, then, with much and close resemblance 

 to the bird, are substantial differences, in an enormous and 

 unbirdlike development of olfactory lobes (with seemingly a 

 covered channel for the olfactory nerve, rare among birds), 

 in evidence of a largely developed and backwardly placed 

 ethmoid and more anterior orbits. Still the resemblance to 

 birds is a true coincidence of functional plan up to a certain 

 point, and altogether different from the resemblance to the bone 

 in the crocodile, which in this point is the most like of 

 reptiles. 



If the bone be compared with the frontal of mammals, pro- 

 bably the bird-like rodents, such as the guinea-pig or rabbit, 

 offer the closest similarity of fonn : the rabbit is to be pre- 

 fen-ed for comparison. But here, though the general form of 

 the bone would be sufficiently Hke to admit of comparison, it 

 will be seen that the eye is situated altogether at the side of 

 the fore part of the cerebrum and large olfactory lobes, which 

 extend between the orbits ; and then the condition of the 

 softer parts of the ethmoid is ver}' different from and not com- 

 parable with the condition of the ethmoid in birds, and unlike 

 any known condition in Omithosaurs. In the interior of the 

 cranial ca^nty of the rabbit, the development of the olfactory 

 lobes comes much nearer to the omithosaurian than any thing 

 seen in birds. Yet olfactory lobes are as much a feature of 



