THE FROG 129 



covering the cartilage just in front of the ethmoid ; the latter are 

 a pair of bones, also in front of the ethmoid, on the ventral surface 

 of the skull, each bone bearing a group of small teeth. 



The upper jaw and the other remaining portions of the visceral 

 skeleton still remain to be described. The upper jaw forms two 

 distinct arches,— an outer or maxillary arch, and an inner or pala- 

 topterygoid arch, the former consisting of three and the latter of 

 two membrane bones on each side. 



The three bones of the maxillary arch, on each side, are the 

 quadratojugal, the small posterior bone ; the maxillary, the long, 

 thin bone which bears most of the teeth ; and the premaxillary, 

 the small anterior bone which forms the anterior end of the skull 

 and also bears teeth. 



The two bones of the palatopterygoid arch, on each side, are the 

 pterygoid and the palatine. The former is a large bone which lies 

 at the hinder end of the skull, medial to the quadratojugal and 

 the maxillary, and is best seen on the ventral surface. It has three 

 ends,— its forward end going to the maxillary bone, its hinder end 

 to the quadratojugal, and its medial end to the parasphenoid. The 

 palatine is a slender bone which lies on the ventral surface and ex- 

 tends from the maxillary, at the forward end of the pterygoid, 

 transversely to the ethmoid. 



The bones and cartilages by which the lower jaw is suspended 

 from the cranium are called the suspensorium. In the frog it is 

 formed on each side by a small cartilage, the quadrate, and a mem- 

 brane bone, the paraquadratum or squamosal. The quadrate lies 

 at the extreme lateroposterior end of the skull, in close connec- 

 tion with the quadratojugal bone and between the pterygoid and 

 squamosal ; the lower jaw articulates with its outer surface. The 

 squamosal is a T-shaped bone which lies on the dorsal surface of 

 the skull in the region of the auditory capsule ; it supports the 

 tympanic membrane. 



Exercise 36. Draw a view of the dorsal aspect of the skull. Show 

 accurately the outlines of the bones and cartilages, and indicate 

 by a difference in color or by shading which are cartilage bones 

 and which are membrane bones. 



Exercise 37. Draw a similar view of the ventral aspect. 



