42 THE FROG CHAP. 



pro-otic of either side enclose a cavity ; in this the organ of hearing is 

 contained. The exoccipital is perforated, just in front of the condyle, 

 by a double aperture (Nv. 9, 10) through which two nerves, the 

 glossopharyngeal and the vagus, pass on their way from the brain. The 

 pro-otic is similarly perforated or notched for the trigeminal and facial 

 nerves (Nv. 5, 7). 



The dorsal surface of the brain-case is covered by two longish, flat 

 bones (FR.PA]. In the young condition each of these consists of two 

 distinct bones, the front one the frontal, the hinder the parietal. As 

 the young frog grows the frontal and parietal of either side becomes com- 

 pletely fused, forming a single fronto-parietal. On the upper surface of 

 each olfactory capsule is a roughly triangular bone, the nasal (NA], in 

 front of which is the corresponding nostril. 



The ventral surface of the brain-case is covered by a single bone 

 (PA.SPH] having the shape of a T. The stem extends forwards in 

 the middle line as far as the olfactory capsules, while the arms stretch 

 outwards beneath the auditory capsules. This very characteristic bone 

 is the parasphenoid. On the under surface of the olfactory capsules, 

 corresponding tc the nasals above, are a pair of irregular bones, the 

 vomers ( VO}. Their outer edges are notched and help to bound the 

 internal nostrils ; their posterior ends bear the vomerine teeth. 



The anterior end of the brain-case is surrounded by a bone 

 (SP.ETH) which extends forwards into the region of the olfactory 

 capsules, and is partly covered by the fronto-parietals above and by the 

 parasphenoid below. This is the girdle-bone or sphenethmoid. In the 

 disarticulated skull it is seen to have a very peculiar shape. Its 

 posterior half encloses a single cavity in which the fore-end of the brain 

 (Fig. 6, olf. 1} is lodged. Its anterior half encloses two cavities, right 

 and left, separated from one another by a vertical partition, and serving 

 to lodge the posterior ends of the olfactory sacs or organs of smell. 

 Each of these cavities communicates with the single posterior cavity by 

 a small hole through which the nerve of smell passes. 



Between the girdle-bone in front and the pro-otic behind, the side- 

 walls of the skull are formed of cartilage perforated by a rounded 

 aperture, the optic foramen (Nv. 2\ for the nerve of sight. 



Forming the outer part of the suspensorium is a hammer-shaped bone, 

 the squamosal (SQ) ; its head is applied to the auditory capsule and 

 projects forwards into the orbit. 



The upper jaw is formed of three bones on either side. In front is the 

 premaxilla (PJlfX), a short bone, sending off an upward process 



