214 



ON THE STRUCTURE OF THE SKULL. 



Like the Carp and the Siluroids, the Amphibia are devoid of 

 any inter-orbital septum, the cranial cavity remaining of tolerably 

 even size from the occipital foramen to its anterior termination. 



In the Frog (Fig. 86) the skull is roofed in by two long flat 

 membrane bones {Pa, Fr), which correspond with the parietals 

 and frontals, and, in fact, each originate in two distinct centres, 



Fig. 86. — Skull of Rana esculenta. Seen A, from above ; E, from below ; C, from the 

 side (after Duges). — x, the parasphenoid ; y, the girdle bone ; z, the " temporo- 

 mastoid " of Dug&s. 



one in front and one behind. In front of these are two other 

 membrane bones (Na, Na), which have been variously inter- 

 preted, but which probably answer to the nasals. On the base 

 of the skull is the long single parasphenoid (#), the hinder part 

 of which is produced into two broad lateral processes, which 

 underlie the auditory capsules. 



AY hen these membrane bones have been stripped off, a sub- 

 jacent cartilaginous cranium becomes apparent, produced behind 

 into two lateral enlargements, or tuberosities, for the auditory 

 organs, and having certain fontanelles or membranous spaces in 

 its upper wall (Fig. 87). In the substance of this cartilaginous 

 cranium, posteriorly, are two ossifications, one on each side of 



