SKULL 



123 



(basibrcmchial) 



posterior nostrils are always situated between the vomers and 

 palatines. The two premaxillse, on the form of which depends 

 that of the beak, are fused, and a maxillopalatine process arises 

 from the maxilla anteriorly. The maxilla and quadrate are 

 connected by a delicate jugal and quadratojugal, and a squamosal 

 is present. Other investing bones are the nasals, frontals, parietals, 

 and lacrymals or prefrontals. 



Teeth were present in Jurassic and Cretaceous Birds (Archae- 

 opteryx, Hesperornis, Ichthyornis), but were no longer developed 

 from the Tertiary period on- 

 wards, their place being taken 

 functionally by horny sheaths 

 covering the bones of the jaws, 

 and thus forming a beak, much 

 as in Chelonians. 



In Meckel's cartilage, two 

 replacing bones are formed, viz., 

 an articular and a mentoman- 

 dibular: the investing bones 

 are a dentary, splenial, coronary, 

 supra-angular, and an angular, 

 and their relations are essenti- 

 ally similar to those seen in 

 Reptiles : they, however, become 

 fused in the adult, and the two 

 rami of the mandible unite 

 distally by synostosis. 



The hyobranchial skeleton 



(Fig. 89) is greatly reduced. FIO. 89. -HYOBRANCHIAL SKELETON OF 

 The median body consists of an FOWL. (After Gegenbaur ; lettering 



entoglossal (basihyoid) passing after Kallius. ) 

 anteriorly into a primarily paired 



paraglossal, which extends into the tongue, and posteriorly into a 

 urohyal (basibranchial). The single pair of cornua belongs to 

 the first branchial arch, and may, as in the Woodpecker, give rise 

 to long, jointed rods extending far over the skull. The columella 

 is the only part of the hyoid which persists, and even in the 

 embryo there is no trace of a second branchial arch. 



branchial nrcli 



Mammals. 



In Mammals, the skull of which in many respects indicates an 

 origin of the Order from reptile-like ancestors, there is a much 

 closer connection between the cranial and visceral regions than is 

 the case in the Vertebrates already described. In the fully- 

 developed skull both maxillary and palatopterygoid regions are 

 closely united to the cranium, so that the facial and cranial portions 



