244 THE ANATOMY OF VERTEBRATED ANIMALS. 



S twopsida, the epiotic and the opisthotic are anohjlosed 

 with the occipital segment before they unite with the prootic. 

 In the primordial skull of the bird the olfactory organs are 

 surrounded by cartilaginous capsules, the lateral walls of which 

 send in turhinal processes of very various degrees of complex- 

 ity. When the posterior wall of this capsule is ossified, the 

 boue thus formed represents the prefrontal, or lateral mass of 

 the ethmoid, of the mammal. It is largely developed in the 

 Apteryx, in the CasuaridcB, and many other birds, but is ab- 

 sent in the Struthio?iidw / and, in other birds, is often repre- 

 sented by a mere bar of bone standing out from the ethmoidal 

 ossification. 



Tlie 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 be- 

 comes undistinguishably fused with the prefrontal. Some- 

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

 size, and sends a process backward beneath the orbit, which 

 may join with a post-orbital process of the frontal, and so cir- 

 cumscribe the orbital cavity. Opisthoconius exhibits the pe- 

 culiarity of the complete anchylosis 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 appear be- 

 low the orbit, parallel with the jugal arch. A post-orbitf 1 

 process may be developed from the frontal, or from the ali- 

 sphenoid ; and, in the latter case, may be separately ossified. 



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

 ly, anchylosed with the other bones. It often sends a process 

 downward 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 Mammalia. 



The premaxillas are modified in a manner which finds a 

 partial parallel only among the JReptilia. They are tri-radiate 

 bones of great size, whicli, usually, give off three processes ; 

 AW ascending process to the frontal ; a palatine process, along 

 tiie middle of the palate, to the palatine bones ; and an exter- 

 nal, or maxillary process, vrhich forms the greater part of the 

 margin of the beak, and unites with the maxilla. The two bones 

 are very early represented by one continuous ossification. 



