582 MANDIBULAR AND HYOID BARS. 



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 subsequently 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 com- 

 pletion 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 hyomandibular of fishes. 



Parker holds that the columella auris of the Anura is the homologue 

 of the hyomandibular. 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 hyomandibular 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 hyomandibular 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 

 development ; so striking indeed as to suggest that the secondary 

 modifications it has undergone are sufficiently considerable to 

 render great caution necessary in drawing morphological con- 

 clusions from the processes which are in some instances ob- 

 servable. 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 



