,.l8o VERTEBEATE SKELETON 



joint between palatine and pterygoid, formed by a part (hemipterygoid) of the 

 pterygoid fusing with the palatine (fig. 193, A) and separating from the pterygoid 

 with a joint between. The palatines have lost their connexion with the maxilloe 

 and now meet and fuse with the premaxillae. Ti)iamus, is between the two 

 groups, but nearer the Dromaeognaths. 



In Ratites the quadrate has a single head (partly divided in a 

 few) for articulation with the otic capsule; in Euornithes it has a 

 double head, one for articulation with the capsule, the other with the 

 squamosal. There is a strong medial and ventral process extending 

 towards the pterygoid, while the bone connects with both pterygo- 

 palatine and zygomatic bars. Its mobility was noted above. 



The lower jaw (fig. 191) has an articulare behind, ossified in 

 Meckel's cartilage and there is a mento-Meckelian at the tip of the 

 jaw. The usual Sauropsidan bones form around the cartilage. The 

 dentalia form most of the jaw, followed medially by one or two 

 splenials. There is a surangulare or coronoid on the upper side 

 which affords insertion for the masseter muscle, while, lower, the 

 angulare, together with the articulare afford places of insertion of 

 the digastric. There is often a vacuity in the side wall of the jaw. 

 The dentalia of the two sides fuse in all modern birds, but in some 

 fossils the two halves of the mandible were apparently movable. 



The hyoid apparatus (fig. 187) is most like that of Chelonia, of 

 existing reptiles, in general appearance, but several points of homol- 

 ogy are uncertain. The structure consists of a long and slender 

 body composed of two copula\ often distinct in the adult (then 

 known as basihyal in front, urohyal behind). In front of the first 

 is a single or paired entoglossum, while between the two copulas a 

 pair of greater cornua are attached. These last are part of the first 

 branchial arch and are usually two- (sometimes three-) jointed. 

 They reach their greatest development in Picid^ and Trochihdae 

 where they curve around the base of the cranium and terminate on 

 its upper surface near the nares. 



If the development described above (p. 173) in MdopsUtaius hold, then the 

 basal parts of the hyoid are lost in the loop in that genus, while the hinder parts 

 of the entoglossum, usually called the hyoid cornua, are formed from the para- 

 glossje, which certainly are not parts of the hyoid arch. The entoglossum is 

 either simple or shows traces of its paired origin, being either two-horned or 

 perforate. The urohval extends far behind the attachment of the greater 

 cornua. 



