io6 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



when the mouth is opened, the temporal arcade aiding in the motion (fig. 109). 

 The vomers may be paired; usually they form a thin vertical plate between the 

 anterior ends of the pterygoids; occasionally they disappear. The choanae 

 are between the palatines and vomers. Some birds have an 'os uncinatum,* 

 a small bone connecting the lacrimal with the palatine or zygomatic bar. All 

 of the bones enumerated on page 73 may appear in the development of the 

 lower jaw. 



Teeth occur only in a few fossil birds, where they are implanted in sockets; 

 several species are known to have a dental ridge in the embryo (see Development 



Fig. 112. — Chondrocranium of a pig, after Mead, as; alisphenoid; cl, posterior 

 clinoid process; cr, fenestra cribrosa; end, foramen for endolymph duct; ftn, foramen 

 magnum; h, fossa hypophyseos; Isr, lateral superior recess; os, orbitosphenoid; pi, 

 parietal lamina; sn, septum nasi; tn, tectum nasi; 2-12, exits of nerves. 



of Teeth). The hyoid apparatus (fig. in) consists of a pair of cornua (first 

 branchials) sometimes extremely long, connected by the hyoid copula (os ento- 

 glossum), behind which is a second copula (urohyal), while in front of|the ento- 

 glossum is a 'paraglossal' element with a pair of small cornua. 



The palatal structures have considerable importance in classification. All 

 living birds can be arranged in two groups. In the 'dromaeognathous' group 

 the palatines and pterygoids do not articulate with the rostrum, the vomers 



