SKULL 355 



The front part of the upper jaw is closely applied to the nasal 

 capsule, but behind it diverges widely, to make a large space, 

 the orbit, between the jaw and the cranium. In the orbit is the 

 eye. 



The primary cartilaginous upper jaw remains unossified at 

 its posterior end as a small quadrate, which articulates with the 

 lower jaw and is held firm above by the squamosal. In front 

 the cartilage partially ossifies to form a pterygoid and a palatine, 

 but in the group of animals to which the frog belongs these carti- 

 lage bones are lost and replaced by membrane bones to which 

 the same names are given. The pterygoid is Y-shaped, with the 

 fork directed backwards, the inner arm abutting on the auditory 

 capsule and the outer on the quadrate, which it helps to hold 

 against the squamosal. The 

 palatine runs transversely 

 from the anterior end of the 

 pterygoid to the spheneth- 

 moid. The cartilage of the 

 jaw runs forward from the 

 quadrate as a continuous ^^^ 



bar on the outside of the F^^- 270.— The skull of frog, seen from 



pterygoid bone and in front 



- , , . . -, co/. Columella; ex., exoccipital ; f.m., foramen mag- 



01 the paiatme. A second num ; o.c, occipital condyle ; pro., pro-otic ; pL. 



r -x -t • pterygoid ; q., quadrate ; q.j. quadratojugal ; 



series 01 membrane bones is sq., srjuamosal; IX.X., foramen for ninth and 



. , • 1 i 1 • • , • tenth cranial nerves. 



situated outside the primitive 



jaw, and borders the opening of the mouth. In contact with 

 the quadrate and squamosal is a small quadratojugal, and from 

 this runs forwards a long maxilla, which bears teeth, and is 

 in contact in front with the nasal capsule and pterygoid. In 

 front of the maxilla is a small premaxilla, also with teeth, which 

 rests on the front of the nasal capsule. The two premaxillcC 

 touch in front. 



The lower jaw or mandible consists of two halves united in 

 front by a ligament. Each half is a curved rod of cartilage, 

 known as Meckel's cartilage, ossified at the anterior end to form 

 the small mentomeckehan bone, and almost completely en- 

 sheathed by a couple of membrane bones, the angulosplenial 

 within, and the dentarv without. The latter does not, as its name 

 would imply, bear teeth, the frog having no teeth in the lower 

 jaw. At the near end or angle of the jaw, the dentary bears a 

 small knob or condyle, which fits into a hollow, on the end of 



