XIII.J THE FROG. 163 



in by membrane, which lies in the inter-orbital region under 

 the parieto-frontals, and by the foramina for the exit of the 

 cranial nerves. It consists entirely of cartilage, except where 

 the exoccipitals, the pro-otics, and the sphenethmoid invade 

 its substance. In front of the septum of the anterior cavity 

 of the sphenethmoid, it is continued forward between the two 

 nasal sacs, as the cartilaginous partition between the nasal 

 cavities {septum narium), from which are given off, dorsally 

 and ventrally, transverse alse of cartilage which furnish a 

 roof and a floor, respectively, to the nasal chambers. Of 

 these, the floor is the wider. The dorsal and ventral alse 

 pass into one another where the chondro-cranium ends 

 anteriorly and give rise to a truncated terminal face, which 

 is wide from side to side, narrow from above downwards, 

 and convex in the latter direction. The lateral angles of 

 this truncated face are produced outwards and forwards 

 into two flattened prce-nasal processes; these widen externally 

 and end by free edges which support the adjacent portions 

 of the premaxillse and maxillse. From the ventral face, 

 just behind the truncated anterior end of the chondro- 

 cranium, spring two slender cartilages, the rhinal processes. 

 Each of these inclines towards the middle line and ends 

 against the middl-e of the posterior face of the ascending pro- 

 cess of the premaxilla by a vertically elongated extremity. 

 An oval nodule of cartilage is attached to the posterior face 

 of the dorsal end of the ascending process of the premaxilla, 

 and serves to connect it with the rhinal process. On the 

 dorsal face of the chondro-cranium, just above the point of 

 attachment of the rhinal processes, the external nasal aper- 

 tures are situated, and the outer and posterior margins of 

 each of these apertures are surrounded and supported by a 

 curious curved process of the cartilaginous ala — the alinasal 

 process. Where the sphenoidal and the ethmoidal portions 

 of the sphenethmoid meet, a stout, transverse, partly osseous 



11—2 



