OHGANIZATION OJT THE OBNITHOSAUBIA. 91 



forward growths both from the squamosal and frontal margins ; 

 and in the Snipe both of these processes meet the lachrymal 

 bone. Thus one distinctive feature of the bird's orbit, by which 

 it differs alike from Ornithosaurs, reptiles, and mammals, is that 

 it forms a circle above the bar containing the malar bone, so that 

 the malar bone is not admitted into the orbit of the eye. 



In the Grround-Hornbill (JBucorvus ahyssinicus), however, and in 

 the Shoebill (-BaZew'eeps re^r), the lachrymal bone meets the malar 

 bar without uniting with it, and the postfrontal region of the 

 skull is prolonged downward almost as far as the malar bar — thus 

 showing that it is possible for a bird to have its orbital circle 

 formed by the same bones, and in the same way, as among Ptero- 

 dactyles : that is, the frontal bone is above, the lachrymal in front, 

 and the malar below. But Ornithosaurs sometimes differ from 

 birds in admitting the quadrato-jugal bone into the orbital circle 

 behind. The quadrato-jugal bone has, I belieye, in most speci- 

 mens hitherto been regarded by others and by ^myself as the post- 

 frontal bone ; btit the postfrontal bone seems to me now usually 

 to have no separate existence in Ornithosaurs, being united with 

 the frontal bone as in adult birds. 



My reasons for making this determination are, that the bone in 

 question appears usually to have two articulations with the qua- 

 drate bone, and to be situated between the malar bone and the 

 proximal end of the quadrate boue. Since the frontal bone 

 expands at the back of the orbit as in birds, and the squamosal 

 bone similarly contributes to the wall of the brain-cavity, there 

 can be no reason for supposing that the bone in question, which 

 makes the outer boundary of the temporal fossa, is the postfrontal, 

 so long as the quadrato-jugal bone remains unaccounted for. 

 The position of the bone is somewhat analogous to that of 

 the quadrato-jugal Satteria, so far as concerns the orbit, and 

 similar to the quadrato-jugal in Iguana in its relations to the 

 quadrate and squamosal bones, and therefore is more lizard-like 

 than the quadrato-jugal of birds. But, in consequence of this ar- 

 rangement, it results that the malar bone unites with the distal 

 end of the quadrate bone ; and this union distinguishes Ornitho- 

 saurs from all existing animals, whether birds or reptiles. These 

 differences from birds, even from a morphological point of view, 

 ought not to be regarded as resemblances towards one class of 

 animals or another, but merely as characters useful in the subor- 

 dinate details of classification. 



