SKULL— OSSIFICATION 



71 



function there is a tendency for at least the anterior part of this 

 cartilage to be connected, directly or indirectly, with the floor or sides 

 of the cranium or to end freely in front, the connexion between the 

 two sides being lost. Its posterior end, except in mammals, still 

 furnishes the part (quadrate) which enters into the hinge of the 

 jaws. 



With ossification a series of bones are formed in this cartilage, 

 the number of which varies in the different classes. In all Verte- 

 brates (mammals excepted) in which the jaws are formed by membrane 

 bones, the hinder end of the cartilage forms a quadrate bone, the 

 upper half of the hinge of the jaws. The more anterior part of the 



Fig. 74. — Mandibular and hyoid arches and operculum of Pleuronectes (Cole and 

 Johnstone, '01). an, angulare; ar, articulare; d, dentale; hm, hyomandibula; ih, interhyal; 

 imc, intermaxillary cartilage; io, interoperculum; mpt, metapterygoid; mspl, mesoptery- 

 goid; o, operculare; po, preoperculum; pi, palatine; pm, premaxilla; pi, pterygoid; 

 q, quadrate; s, symplectic; 50, suboperculum. 



pterygoquadrate may ossify as several bones. In Teleosts (fig. 74) 

 a part in front of and dorsal to the quadrate forms a metapterygoid 

 bone, while in a few bony fishes its extreme anterior end ossifies as 

 an autopalatine. A few groups have an epipterygoid (columella 

 cranii of lizards, ascending process of Amphibia) which extends up 

 from the rest of the pterygoid structures to the alisphenoid cartilage 

 or to the overlying parietal bone (fig. 75). 



In the primitive lower jaw (Meckehan) there are at most but two 

 ossifications on either side, and even these do not always develop. 

 At the posterior end, each Meckelian may ossify as an articulare 

 which articulates with the quadrate, forming the lower half of the 

 hinge. The other cartilage bone is a mentomeckelian, best known 

 in Anura, at the symphysis of the lower jaw. (There is some evi- 



