IV. VERTEBRATA Hil 



nants of a gill arch. Other gill arches contribute to laryngeal cartilages, 

 the epiglottis and the cartilages of the auditory meatus. 



The anterior members of the visceral skeleton (pterygoquadrate, 

 Meckelian, and hyomandibular) become developed further, but lose more 

 and more their individuality and unite with the cranium; in the mammals 

 forming the 'bones of the face.' It is therefore a source of additional bones 

 which are difficult to follow from class to class, since they change in their 

 functions and consequently in shape and relative size. 



All vertebrates with bony visceral skeleton (figs. 517, 547) have two 

 pairs of membrane bones, right and left, in front of the pterygoquadrates, 

 the premaxillaries (intermaxillaries) and maxillaries. They usually 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 pterygoquadrates are thus forced backwards and 

 form a second series of bones, parallel to the maxillary series, which like- 

 wise may bear teeth. This row of bones consists of an anterior palatine 

 portion and a posterior quadrate part. The cartilages of the palat im- 

 part largely disappear and are replaced, in front, by a pair of comers fol- 

 lowed by a pair of palatines, while farther back are a pair of ptcrygoids. 

 The quadrate portion ossifies into the quadrate bone, which, except in mam- 

 mals, afford the articulation for the lower jaw. The ossifications for the 

 lower jaw occur in a similar way; in front a series of membrane bones, of 

 which the dentary is most important, surrounding Meckel's cartilage, 

 while the hinder part of the Meckelian ossifies into the artimlare, so called 

 because it articulates with the quadrate. The hyomandibular forms 

 only one constantly present bone known by the same name. 



All vertebrates with terrestrial habits have a sound-conducting ap- 

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

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

 lar, the quadrate, and the articulare, to which is added another element, 

 the stapes, which occupies the fenestra ovalis (p. 479) and is derived from 

 the otic capsule itself. In Anura, reptiles, and birds the hyomandibular 

 apparently gives rise to an element, the colmnclln, 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 converted into 

 part of the sound-conducting apparatus, the quadrate forming the inms, 

 and the articulare the 'malleus (figs. 533, 534). Since the lower jaw in this 

 way loses its articulation, a new one is formed by a process from the mem- 

 brane bones which articulates with a membrane bone, the squamosal, 

 to be mentioned below. 



