358 CLASSIFICATION OF VERTEBRATES. 



The cribrosa, together with the superior turbinal bones in the 

 nasal passage, apparently represents the prefrontal of lower 

 vertebrates. 



The nasal cavity is bounded externally by the nasal bones, 

 which are small in cetacea and fused in the old-world apes. 

 Inside the cavity, besides the superior turbinals already referred 

 to, are the 1 inferior turbinals which, beginning as separate ossi- 

 fications, fuse with the maxillaries. The septum of the nose, 

 established by the mesethmoid, is continued by the vomer, in 

 which the paired bones of the lower forms are fused, and are 

 entirely cut off from the roof of the mouth. 



Among the most characteristic features of the facial skele- 

 ton are the close union of the maxillopalatine region with the 

 rest of the skull, and the suspension of the lower jaw direct from 

 the temporal bone without the intervention of a quadrate. The 

 premaxillse may fuse with the maxillae, while maxillae and pala- 

 tines send off horizontal palatine processes, which, meeting in 

 the middle line, form the hard palate, bound the nasal cavities 

 below, and carry the choana far back. The pterygoids may also 

 contribute to the hard palate (some edentates). 



The lower jaw consists of but a single bone, the dentary, on 

 either side, or the two halves may anchylose at the symphysis. 

 In the middle ear are three small bones which form a sound- 

 conducting apparatus leading from the tympanic membrane to 

 the fenestra ovalis. In order from outside in these are the mal- 

 leus, incus, and stapes. Concerning their homologies the most 

 diverse views are held, both incus and malleus having been 

 regarded as the missing quadrate. The views of the homologies 

 given on p. 1 59 seem to be in full accord with the results of 

 the studies of most students who have approached the subject 

 from the embryological standpoint. 



In the mammals the brain exercises great influence upon 

 the shape of the skull. As it nearly fills the cranial cavity, 

 increase in its size can only be accommodated by an outgrowth 

 of the cranial walls. To measure the extent of this outgrowth 

 and thus approximately to obtain an index of cerebral develop- 

 ment, the facial angle is employed. According to the system 

 of Camper this is the angle formed by two lines, one passing 



