parasphenoid frontoparietal 

 orbifosphenoid 

 orbitonosol foramen 

 prefrontal 

 vomer 

 septomaxillo Jc * vU \.i^. \ ^.^^W / 



frontoparietal ^' ^' ^" prootic 

 orbifosphenoid Jv--A---+-4^ ,endolymphotic duct 



exoccipital 



metotic Fissure 

 IX-X-XI 



perilymphatic 

 fenestra 

 basioccipital 



articular 



Figure 4-21. Skull and mandible of tfie Bullfrog. A, lateral view of skull and mandible; B, dorsal 

 view of skull; C, ventral view of skull; D, mediol view of right half of endocranium; E, medial view 

 of right half of mandible. 



The pterygoid sutures along much of the length of the 

 quadrate process of the squamosal. The quadrate is much 

 reduced and limited to the articulation area. The small 

 quadratojugal is fused to the quadrate. A broad parasphe- 

 noid covers the ventral aspect of the cranium and e.xtends 

 forward between the orbits. 



The endocranium has exoccipital, prootic, and orbito- 

 sphenoid ossifications only. The exoccipital invades the 

 area of the opisthotic, but in the Bullfrog there is evidence 

 of a separate ossification center for this opisthotic portion. 

 The orbitosphenoid ossification lies anterior to the optic 

 nerve and as such might be viewed as an ethmoid. In the 

 primitive amphibian this bone extended both in front of and 

 behind the optic nerve. There is a calcified operculum be- 

 hind the stapes in the lateral wall of the otic capsule. The 

 operculum is an irregular mass as closely attached to the 

 stapes as to the capsule wall. 



The premaxilla and maxilla bear slim pointed teeth, 

 which are like those of the salamander in that the point is 



easily broken off, along an abscission line, from the base. 

 The teeth are attached in the pleurodont fashion to the jaw. 

 The mandible is without teeth. It is formed of a small 

 dentary, a large prearticular that makes up most of the 

 ramus and is fused to the small calcified articular, and a 

 small mental ossification on either side of the symphysis. 



The hyoid and branchial arches give rise to a columella 

 and the hyoid apparatus. The latter is a completely carti- 

 laginous structure formed of a broad copular plate attached 

 to the cranium through the hyoid arch. The epihyal is 

 attached to the cranium behind the area of contact of the 

 basicranial process of the pterygoid and below the fenestra 

 vestibuli and the columella. 



EMBRYOLOGiCAL DEVELOPMENT The development of the 

 frog head skeleton is of interest because of the early appear- 

 ance of larval jaws lying anterior to the definitive jaws 

 (Figure 4-22). These larval jaws suggest a way that the jaws 

 of the gnathostomes might have arisen in phylogeny. This 



OTHER TETRAPODS 



83 



