IV. VERTEBRATA. 525 



the cranium; in the mammals forming the ' bones of the face/ It 

 is therefore a source of additional bones which are difficult to fol- 

 low from class to class, since they change in their functions and 

 consequently in shape and relative size. 



All vertebrates with bony visceral skeleton (figs. 561, 589) 

 have two pairs of membrane bones, right and left, in front of the 

 pterygoquadrates, the premaxillaries (intermaxillaries) and max- 

 illaries. They bear, in toothed vertebrates, the marginal row of 

 teeth, which are distinguished from the palatopterygoid teeth in 

 that they are opposed by the teeth of the lower jaw. The ptery go- 

 quadrates are thus forced backwards and form a second series of 

 bones, parallel to the maxillary series, which likewise may bear 

 teeth. This row of bones consists of an anterior palatine portion 

 .and a posterior quadrate part. The cartilages of the palatine part 

 largely disappear and are replaced, in front, by a pair of vomers 

 followed by a pair of palatines, while farther back are a pair of 

 pterygoids. The quadrate portion ossifies into the quadrate bone, 

 which affords the articulation for the lower jaw, The ossifications 

 lor the lower jaw occur in a similar way; in front a series of mem- 

 brane bones, of which the dentary is most important, surrounding 

 Meckel's cartilage, while the hinder part of the Meckelian ossifies 

 into the articulare, so called because it articulates with the quad- 

 rate. The hyomandibular forms only one constantly present bone 

 known by the same name. 



If all vertebrates with bony skeletons be compared, it is found 

 that those with terrestrial habits have a sound-conducting apparatus 

 in connexion with the ear. This is composed of elements which, 

 in the fishes, lie in the neighborhood of the otic capsule, the 

 hyomandibular, the quadrate, and the articulare, to which is 

 added another element, the stapes, which occupies the fenestra 

 ovalis (p. 544) and is derived from the otic capsule itself. In 

 Anura, reptiles, and birds the hyomandibular apparently gives 

 rise to an element, the columella, which abuts against the stapes. 

 In the mammals stapes and columella are possibly fused, while 

 quadrate and articulare undergo a change of function, losing 

 their position in connexion with the articulation of the jaws and 

 being COD verted into part of the sound-conducting apparatus, the 

 quadrate furnishing the incus, the articulare the malleus (figs. 

 576, 577). 1 Since the lower jaw in this way loses its articulation, 

 a new one is formed by a process from the membrane bones. 



According to this view the lower jaw of a mammal is not equivalent 

 to the lower jaw of a bird, since in the latter the hinge is furnished by the 



