184 ELEMENTS OF ORNITHOLOGY. 
anchylose, or ossify, together, as will be further mentioned 
under the head of ‘* Development.” 
The conditions above described being those common to the 
Class, a few of the more important variations in the form of the 
bones of the skull may now be noted. 
The Jachrymal is sometimes indistinguishable from the adja- 
cent prefrontal. It may be greatly elongated and even unite 
witli the postorbital process, thus forming, with the help of the 
other bones, a complete ring round the margin of the orbit. 
A bony bar may even extend between and unite the post- 
frontal and squamosal processes, as in some Parrots. 
The nasal bones, in other instances, may be completely 
anchylosed with the lachrymals, as in Opisthocomus. 
The vomer is generally single, but may be double (side by 
side), as in the Woodpecker. It may be more or less obsolete, 
as in the Pigeon and Duck ; or large and broad, as in the Tina- 
mou; and it may be deeply cleft behind and abruptly truncated 
in front. 
The maxilla may vary much as to size, and generally sends 
inwards a process, or plate, of bone called the maaillo-palatine 
process (fig. 152, mvp), and this may be of a spongy nature. 
It may unite with its fellow of the opposite side, or may be not 
only distinct from that, but from the vomer also. Each palatine 
unites with the premaxilla, either by bony union, suture, or by a 
flexible joint—as in Parrots. In passing to this junction it tra- 
verses the ventral side of the just mentioned process of the maxilla. 
Instead of directly articulating with the rostrum posteriorly, it 
may be separated from it by the vomer, or, as in the Ostrich, 
pass back directly to the pterygoid, hardly even approximating 
to the rostrum. 
The pterygoid may, as in the Ostrich, pass outwards and 
backwards to the quadrate, not from the palatine and the 
rostrum, but from the palatine and a process of the basi- 
sphenoid behind the rostrum. The processes with which the 
pterygoids articulate, whether such processes proceed from the 
basisphenoid or from the rostrum, are known as basipterygoid 
processes. 
These varying conditions of the bones of the skull need sepa- 
rate description when the characters of separate groups of birds 
come to be noticed. It may be well, however, here to note 
certain terms which have been applied to some leading moditi- 
cations of the parts of the facial skeleton of Birds. Thus a skull 
