22 THE VERTEBRATE SKELETON. 



dorsal portion, the palato-pterygo-quadrate bar, which 

 forms the primitive upper jaw and enters into very close 

 relations with the cranium, and a ventral portion, Meckel's 

 cartilage, which forms the primitive lower jaw. The carti- 

 laginous rudiments of both these portions disappear to a 

 greater or less extent and become partly ossified, partly re- 

 placed by or enveloped in membrane bone. 



The posterior part of the palato-pterygo-quadrate bar be- 

 comes ossified to form the quadrate, the anterior part to form 

 the palatine and pterygoid, or the two latter may be formed 

 partially or entirely of periosteal bone, developed round the 

 cartilaginous bar. Two pairs of important membrane bones, 

 the premaxillae and maxillae form the anterior part of the 

 upper jaw, and behind the maxilla lies another membrane 

 bone, the jugal or malar, which is connected with the quadrate 

 by a quadratojugal. The premaxillae have a large share in 

 bounding the external nasal openings or anterior nares. 



In lower vertebrates the nasal passage leads directly into 

 the front part of the mouth cavity and opens by the posterior 

 nares. In some higher vertebrates, such as mammals and 

 crocodiles, processes arise from the premaxillae and palatines, 

 and sometimes from the pterygoids, which meet their fellows 

 in the middle line and form the palate, shutting off the nasal 

 passage from the mouth cavity and causing the posterior nares 

 to open far back. 



The cartilage of the lower jaw is in all animals with 

 ossified skeletons, except the Mammalia, partly replaced by 

 cartilage bone forming the articular, partly overlain by a 

 series of membrane bones the dentary, splenial, angular, 

 supra-angular and coronoid. In many sharks large paired 

 accessory cartilages occur at the sides of the jaws ; and in a 

 few reptiles and some Amphibia, such as the Frog, the ossified 

 representative of the anterior of these structures occurs 

 forming the mento-meckelian bone. In mammals the lower 

 jaw includes but a single bone. 



