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 sphenetkmoid) 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 (squamosafy, 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- 



