184 ELBMBNTS OP OENITHOLOGT. 



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 lachrymal is sometimes indistinguishable from the adja- 

 cent prefrontal. It may be greatly elongated and even unite 

 with 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 maxillo-palatine 

 process (fig. 152, mxp), 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 maxiUa. 

 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 hasipterygoid 



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 modifi- 

 cations of the parts of the facial skeleton of Birds. Thus a skull 



