88 VERTEBRATE ZOOLOGY 



of the auditory capsule ; it is at the level of the hinder end of the 

 frontal bones. Within this bone is the membranous labyrinth of 

 the ear. Both these bones appear on both the dorsal and the 

 ventral side of the skull and are partly covered by the parasphenoid 

 bone and the parietal bones. Between these two bones on the 

 ventral side of the skull there appears a small round bone, the 

 operculum, which exactly fits into a round opening in the auditory 

 capsule called the fenestra ovalis ; a short projection of this bone 

 is directed forward and upward. 



Along the entire outer edge of the auditory capsule is a flat 

 membrane bone, called the paraquadrate or squamosal, from the 

 middle of which a short projection extends to the operculum. 



The remaining bones of the skull belong to the visceral skeleton 

 and may be divided into two groups, the upper jaw and the sus- 

 pensorium of the lower jaw. 



The upper jaw is very weak in Necturus. In the typical amphib- 

 ian it consists of two distinct arches : an outer, or maxillary, and 

 an inner, or palatopterygoid, arch ; in the frog the arches are well 

 represented. In Necturus they are present only in part. The 

 outer arch is represented only by the premaxillary bones. These 

 are a pair of V-shaped membrane bones which form the ante- 

 rior end of the skull and bear the anterior row of teeth. Maxil- 

 lary and jugal bones, which in other amphibians lie back of 

 these and complete the arch, are not present. The inner arch 

 is represented only by the palatopterygoid bones, a pair of large 

 membrane bones which lie immediately back of the vomers and 

 bear teeth at their forward ends. 



The suspensorium is formed by the quadrate cartilage, the quad- 

 rate bone which develops in the cartilage, and the squamosal, or 

 paraquadrate, a membrane bone. The quadrate bone forms the 

 extreme lateral portion of the skull ; its anterior face is the surface 

 with which the lower jaw articulates. The quadrate cartilage ex- 

 tends medially from the bone to the cranium. The paraquadrate, 

 as we have already seen, lies along the side of the skull, extending 

 from the quadrate bone back to the opisthotic. 



Extending laterally from the cranium at the level of the for- 

 ward end of the palatopterygoid bone, on each side, is a small 



