IV. VERTEBRz\TA 4G1 



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 dif&cult 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 premaxiUaries (iiilcrmaxillaries) 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 Ijy 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 palatine 

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

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

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

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

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

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

 while the hinder part of the Meckelian ossifies into the artici/lare, 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 tlae 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 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 converted into 

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

 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. 



