232 THE BIOLOGY OF THE FROG chap. 



skull. Along the middle line they are united by the sagittal 

 suture. Each represents two bones, a frontal and a parietal, 

 and in the early stages of the development of the skull these 

 elements are separate, but subsequently they fuse into a 

 single bone. 



The anterior end of the cranium is surrounded by a bony 

 ring, the ethmoid (or sphenethmoid) bone. This is over- 

 lapped by the fronto-parietals above and the parabasal be- 

 low, and is separated from the prootics behind by quite a 

 long interval of unossified cartilage. The anterior part of the 

 ethmoid is widened out and divided into two chambers by a 

 median vertical partition. The expanded portion forms the 

 posterior wall of the nasal cavity ; the latter may be seen to 

 communicate with the cranial cavity by a pair of small open- 

 ings through which the olfactory nerves pass. The rest of 

 the nasal capsules are formed mainly by cartilage. 



The nasals are two narrowly triangular bones, lying above 

 the nasal capsules ; their bases, which lie near each other in 

 the middle line, are separated from the fronto-parietals by a 

 small part of the roof of the ethmoid. 



The vomers lie ventral to the nasal capsules ; each has 

 three outer processes, between the two posterior of which 

 occur the internal nares; the ventral surface bears the 

 vomerine teeth. 



Suspensorium and Jaws. — The jaws are attached to the 

 cranium by means of an intermediate suspensory apparatus 

 in which the following separate bones are to be distin- 

 guished : — 



(i) The tympanic (squamosal), a T-shaped bone, the main 

 limb of which extends outward and backward to the angle 

 of the jaws ; the posterior end of the cross piece articulates 

 with the prootic, while the anterior end extends obliquely 

 downward in front. Below the tympanic lies (2) the ptery- 



