260 AVES CLASS iv 



lachrymal is often present at the anterior margin of the orbit, and there is an 

 ossified or membranous interorbital septum of considerable size. In the anterior 

 prolongation of the latter is a vertically placed unpaired ethmoid, which is 

 continued in front as a bony or cartilaginous nasal septum. The lateral 

 ethmoids are placed between the orbits and external nares, and are pierced 

 for the passage of the olfactory nerves. The greater part of the beak is 

 formed by the fused premaxillae, with the sides of which the maxillae are 

 fused ; its margins are sheathed with a horny layer, and it is often movably 

 connected with the skull. When the mouth is oj^ened, the end of the beak is 

 raised by pressure transmitted to it from the quadrate by means of the ptery- 

 goid and palatine bones, and more especially by the rod-like quadrato-jugal. 



Broadly speaking, the bones of the palatal portion of the skull are arranged 

 on two plans. In the one the vomer is broad and unites in front with the 

 maxillo-palatines, while behind it receives the posterior extremities of the 

 palatines and the anterior ends of the pterygoids, which are thus excluded 

 from contact with the sphenoidal rostrum. This is the dwmaeognathoiis type 

 of palate found in struthious birds, the apteryges and tinamous. In birds 

 with this arrangement the head of the quadrate usually has but a single 

 articular face, or is but faintty divided into two portions ; and rather long 

 basipterygoid processes from the sides of the sphenoid give this bone some- 

 what of a cruciform shape. 



In the second type of palate, which may be called the euornithic, the vomer 

 embraces posteriorly the sphenoidal rostrum between the palatines, and these 

 latter articulate with the pterygoids and with the sphenoid. This arrange- 

 ment predominates in the vast majority of birds, and they also have the head 

 of the quadrate double, or with two articular facettes. Basipterygoid processes 

 are present in some euornithic skulls, but these most frequently assume the 

 form of low facettes on the sphenoidal rostrum, with which the pterygoids are 

 movably articulated. The euornithic type of skull is subject to various 

 modifications in the development of the vomer and those inwardly directed 

 processes of the maxillae termed the maxillo-palatines. "When the vomer is 

 pointed in front and entirely free from the maxillo-palatines, and these are 

 free from each other, the skull is termed schizognathous ; when the maxillo- 

 palatines are expanded and fused with each other, the vomer being small 

 or absent, the skull is desmognathous ; when the vomer is expanded in front 

 and free from the maxillo-palatines, and these are slender at their point of 

 origin and disjoined, the skull is said to be aegithognathous. 



Two terms applied to conditions of the bones bounding the narial openings 

 and much used in classification, holorhinal and schizoriiinal, may be explained 

 here. In the holorhinal type the openings are more or less oval, the posterior 

 border curved and lying in advance of the posterior ends of the premaxillaries. 

 In the schizorhinal type the openings are more or less elongate with the 

 posterior border angular or slit-like and lying back of the posterior ends of 

 the premaxillaries. 



In all modern birds the mandibular rami become fused at an early stage 

 into a long symphysis, and only among certain Mesozoic forms {Ichthywnis) 

 are they united by suture or by ligaments as in reptiles. The six elements 

 of which the mandible is composed fuse into a single piece as in modern birds, 

 and there is frequently a lateral vacuity between the dentary and splenial, as 

 in crocodiles. Althoutrh the earliest known birds have conical thecodont 



