SKULL AMPHIBIA 121 



bounded behind by the antorbital process, its roof formed by growth from the 

 other parts. Internally it is complicated by structures separating the olfactory 

 and vomero-nasal (Jacobson's) organs, the latter known in no fishes, but common 

 in Tetrapoda. 



The pterygoquadrate is reduced in most Urodeles, less so in Gymnophiona 

 and least in Anura. Except in Gymnophiona the quadrate part connects with 

 the otic capsule by otic and basal processes, while most Urodeles have an epiptery- 

 goid (ascending) process from quadrate to trabecula, dividing the foramen 

 lacerum into two foramina for the fifth and seventh nerves respectively. The 

 relations of the pterygoid process are more primitive in Anura than in the others, 

 as it reaches and fuses with the anterior cranial wall at about the level of the 

 ethmoid region. In Gymnophiona, so far as known, it does not extend so far, 

 while in Urodeles {Ranodon and larval Cryptobranchus excepted) it is practically 

 undeveloped, at least in the larvae. Hence the autostyly of Anura is the more 

 primitive, the others cases of degeneration. 



The Meckelian cartilage needs no description. There are a hyoid and at 

 most four gill-arches (the fifth branchial possibly being represented in the 

 laryngeal cartilages). The hyoid arch is the largest, the others diminishing in 

 size successively and undergoing more or less change at metamorphosis. At 

 most only the hyoid and first two branchial arches are connected by copulae, the 

 arches farther back being connected directly to those in front. The parts of 

 the arches are few, the hyoid having cerato- and hypohyal, the gill-arches cerato- 

 and hypobranchial which are all but universally present in the adult. 



The striking features of the adult skull are its great flatness 

 (except in Gymnophiona, where, in correlation with the burrowing 



Fig. 127. — Base of cranium of Maslodonsaiirns (Fraas, '09). b, basioccipital; c, 

 condyle; eo, exoccipital; ep, tabulare (epiotic);/, foramen magnum; pi. pterygoid; q, 

 quadrate; qj, quadratojugal; so, dermoccipital; sq, squamosal. 



habit, it is more cylindrical); the two exoccipital condyles; the all 

 but invariable absence of ossified basi- and supraoccipitals; the 

 firmly fixed (moniinostylic) quadrate; and in all except Stegocephals, 

 the very great reduction of bones, both in the cranium and the lower 

 jaw, the latter containing, at most, distinct dentale, 'angulare' 

 and splenial (membrane) and a cartilage articulare. The angulare 

 extends forwards on the medial side of Meckel's cartilage, which 

 suggests that it may be a goniale. 



Stegocephalia. — Since development is unknown, the cartilage 

 or membrane character of some bones can be settled only by position 



