SKULL OF VERTEBRATA. 457 



bones for the first time. At the base of the skull is the parasphenoid 

 (Ps), which still retains very much the same characters as it had in 

 Fishes, and in front of this, and in the ethmoidal region, there is a 

 paired bone (vo), which is regarded as the vomer. 



The palato-quadrate is more simple in character than in Fishes. 

 The whole piece sometimes remains for the most part in a cartilaginous 

 condition. An ossification at the point where' it articulates with the 

 lower jaw corresponds to the quadrate of Fishes. In many, the 

 palato-quadrate is divided into an anterior and a posterior portion 

 (Triton). It is not completely united with the cranium, for there is 

 a distinct articular surface on its lower portion, between it and the 

 cranial capsule (Rana). 



There are two covering bones on the palato-quadrate cartilage ; 

 the upper one (Ty) is distinguished, in the Frog, by the possession 

 of a strong process, which is directed forwards, and which probably, 

 though by no means certainly, corresponds to the squamosal of 

 Fishes. As it aids in supporting the tympanum, it may be called 

 the tympanic. The lower bone is the pterygoid (Ft), and it extends 

 forwards along the cartilaginous arch. Its anterior end reaches as 

 far as the palatine, which lies transversely behind the vomer (PI). 

 In some of the Amphibia another bone is continued forwards in 

 front of the glenoid cavity ; this is the so-called jugal (quadratojugal). 



The premaxillas (Px) and maxilla (Mx) appear as covering bones 

 of the primordial cranium ; relations which obtain in many Fishes 

 lead to this condition. The maxilla varies greatly in the extent 

 to which it is developed laterally ; in the Anura it ordinarily extends 

 as far back as the jugal. The premaxilla is connected with the 

 primordial cranium by a process which projects forwards in the 

 middle of the nasal region. 



These maxillary bones did not primitively bound the opening of 

 the mouth, as the presence of special cartilages (rostral and adrostral) 

 in front of the primordial cranium of the larvae of Anura distinctly 

 shows. 



In the lower jaw the primordial cartilage is present, as in Fishes, 

 and bony parts are developed in connection with it, which essen- 

 tially correspond to those of Fishes. 



Parker, W. K., Development of the Skull in the Frog. Philos. Trans. 1871.— 

 Wiedersheim, E., Das Kopfskelet der Urodelen. Morphol. Jahrb. III. 



§ 317. 



The skulls of the Sauropsida have much in common with one 

 another, whilst they are far removed from the skulls of the Am- 

 phibia, and from those of the Mammalia. 



The primordial cranium, as a rule, has the roof incomplete, but 

 it is much more completely ossified than in the Amphibia, while the 

 great size of the bones which are developed in, and from, the palato- 

 quadrate cartilage leaves but a small portion of the true cranium 



