428 



AVES. 



tures are near together on the base of the skull. The lower jaw 

 contains, as in other Sauropsida, six osseous elements, viz., 

 articular, angular, surangular, coronoid, dentary and splenial. 

 The hyoid hone is prolonged into a posterior rod and consists of 

 three basal pieces, of which the anterior is called the entoglossal 

 bone (Fig. 238, Ent), the middle the basihyal (Co) and the 

 posterior rod the urohyal. It carries one pair of cornua {Zh), 

 which are the homologues of the &st branchial arches ; these are 

 usually two-jointed and not connected with the skull, but in 

 most woodpeckers they are much elongated and arch over the 

 skull as far as the forehead, constituting in connection with the 



muscles of their sheath a mechanism 

 for the protrusion of the tongue. The 

 columella auris consists of an osseous 

 rod, the inner end of which fits into 

 the fenestra ovalis while the outer end 

 expands into a triradiate cartilage 

 which is attached to the tympanic 

 membrane. 



The above are the main features in the 

 a\'ine skiiU. The following additional podnts 

 may be noted. The foramen magnum looks 

 downwards as well as backwards. All the 

 occipital bones enter into the foramen mag- 

 num, but the condyle is formed almost en- 

 tirely by the basioccipital. The epiotic and 

 opisthotic fuse with the occipital bones before 

 Fir,. 238.— H void bone of Corr.M they unite with the prootic. Basisphenoids 

 hvS ^Tcomual' /S e'nto: and ahsphenoids are well developed, but 

 glossal bone. the presphenoid and orbitosphenoids, which 



enter into the interorbital septvim are 

 often imperfect. The interorbital septmn (to a varying extent 

 cartilaginous) is formed anteriorly by the mesethmoid {Et), which is 

 continuous in front with the (mainly) cartilaginous intemasal septtim,* 

 and may appear on the surface of the skull between the nasals and fron- 

 tals. It is underlaid by the basisphenoidal rostrum. The tvirbinals or 

 lateral ethmoids are poorly developed. The lacrymals (L) are large and 

 perforated by the lacrymal canal. The nasals {N) are well developed 

 and form the upper and lateral boundaries of the external nares.f The 



* When the intemasal septum is complete the nostrils are said to be 

 impervious {nares imperviae), when it is incomplete they are described as 

 pervious (n. perviae). 



f The term holorhinal is applied to those cases in which the external 

 narial opening is oval, the posterior border being curved and in front of 

 the posterior end of the premaxillae. In the so-called schizorhinal arrange- 

 ment the openings are elongated, the posterior border being angular or 

 slitlike and behind the posterior ends of the premaxillae. 



