THE SKULL IN BIEDS. 285 



capsule is ossified, the bone thus formed represents the 

 prefrontal, or lateral mass of the ethmoid, of the mammal. 

 It is largely developed in the Apteryx, in the Casuaridce, 

 and many other birds, but is absent in the Struthioniclce ; and, 

 in other birds, is often represented by a mere bar of bone 

 standing out from the ethmoidal ossification. 



The lachrymal is, usually, a distinct and large bone articu- 

 lated with the nasal and frontal above, with the prefrontal 

 internally, and with the maxilla, below ; but sometimes it 

 becomes undistinguishably fused with the prefrontal. Some- 

 times, on the contrary, as in the Parrots, it acqtiires a large 

 size, and sends a process backwards beneath the orbit, 

 which may join with a post-orbital process of the frontal, 

 and so circumscribe the orbital cavity. Opisthocomus ex- 

 hibits the peculiarity of the complete ankylosis of the nasal 

 with the lachrymal, which is quite free from the frontal 

 and moves with the hinged beak. A supra-orbital bone, or 

 chain of bones, may be developed in connection with the 

 orbital margin of the frontal bone ; and, occasionally, infra- 

 orbital bones, apj^ear below the orbit, parallel with the jugal 

 arch. A post-orbital process may be developed from the 

 frontal, or from the alisphenoid ; and, in the latter case, 

 may be separately ossified. 



The squamosal is closely applied to the skull, and is, 

 usually, ankylosed with the other bones. It often sends a 

 process downwards over the quadrate bone, and it may be 

 united by bone with the post-orbital process of the frontal, 

 as in the Fowl. 



The frame of the tympanic membrane not unfrequently 

 contains distinct ossifications, which represent the tympanic 

 bone of the 3£ammalia. 



The premaxillge are modified in a manner which finds 

 a partial parallel only among the Reptilia. They are 

 tri-radiate bones of great size, which, usually, give off 

 three processes; an ascending process to the frontal; a 

 palatine process, along the middle of the palate, to the 

 palatine bones; and an external, or maxiUary process, 

 which forms the greater part of the margin of the beak, 



