THE SKULL. 479 



Amphibia. The development of the autostylic piscine skulls 

 has unfortunately not yet been studied ; and the most primitive 

 autostylic types whose development we are acquainted with are those 

 of the Amphibia ; on which a large amount of light has been shed 

 by the researches of Huxley and Parker. 



The modifications of the hyoid arch are comparatively simple and 

 uniform. It forms a rod of cartilage, which soon articulates in front 

 with the quadrate element of the mandibular arch, and is subse- 

 quently attached by ligaments both to the quadrate and to the 

 cranium. In those Amphibia in which external gills and gill clefts 

 are lost, it fuses with the basal element of the hyoid (fig. 330), 

 which, together with the basal portions of the following arches, 

 forms a continuous cartilaginous plate. On the completion of these 

 changes the paired parts of the hyoid arch have the form of two 

 elongated rods, known as the anterior cornua of the hyoid, which 

 attach the basihyal plate to the cranium behind the auditory capsule. 



It is still uncertain whether there is any distinct element corresponding 

 to the hyoraandibular of fishes. 



Parker holds that the columella aims of the A mini is the homologue 

 of the liyomandibular. The columella develops comparatively late and 

 independently of the remainder of the hyoid arch, but the similarity 

 between its relations to the nerves and those of the liyomandibular is 

 put forward by Parker as an argument in favour of his view. The 

 early ligamentous connection between the quadrate and the upper end of 

 the primitive hyoid is however an argument in favour of regarding the 

 upper end of the primitive hyoid as the hyoniandibular element, not 

 separated from the remainder of the arch. 



The history of the mandibular arch is more complicated than 

 that of the hyoid. The part of it which corresponds with the upper 

 jaw of Elasmobranchii exhibits most striking variations in develop- 

 ment ; so striking indeed as to suggest that the secondary modifica- 

 tions it has undergone are sufficiently considerable to render great 

 caution necessary in drawing morphological conclusions from the pro- 

 cesses which are in some instances observable. A more satisfactory 

 judgment on this point will be possible after the publication of a 

 'memoir with which Parker is now engaged on the skulls of the 

 different Anura. 



The membrane bones applying themselves to the sides of the 

 mandibular arch are relatively far more important than in the lower 

 types. This is especially the case with the upper jaw where the 

 maxillary and premaxillary bones functionally replace the primitive 

 cartilaginous jaw ; while membranous pterygoids and palatines apply 

 themselves to, and largely take the place of, the cartilaginous palatine 

 and pterygoid bars. 



Two types worked out by Parker, viz. the Axolotl and the com- 

 mon Frog, may be selected to illustrate the development of the 

 mandibular arch. 



In the Axolotl, which may be taken as the type for the Urodela, 



