156 ANIMAL BIOLOGY. [Part I. 



the frontal segment there are no separate sphenoid or ethmoid 

 bones ; the curious girdle-bone sphenethmoid (s. e.) represents 

 these skull-elements. The frontals are represented by the 

 anterior portions of the large parieto-frontals. The front wall 

 of the brain-case is formed by the sphenethmoid, a dice-box- 

 shaped bone pierced by the olfactory nerves. In the side walls 

 the prootics (pr. o.) are large well-ossified bones; but the squa- 

 mosal takes no share in closing in the brain-case. In the absence 

 of other basal elements, the para-sphenoid (pa. sp.) underlies and 

 strengthens the cartilaginous floor of the skull. This is a con- 

 spicuous dagger-shaped bone, the blade of which probably 

 answers to the rostrum in the fowl, while the crossguard is 

 perhaps homologous with the large basi-temporal mass. 



Thus in the brain-case of the frog we have the paired parieto- 

 frontals above, the single para-sphenoid below, the sphenethmoid 

 in front, the prootics at the sides, and the ex-occipitals behind. 



The positions of some of the nerve exits is seen in 50, B. 

 The foramen for the ninth and tenth passes through the ex- 

 occipital mass. That for the fourth or pathetic is very minute 

 above and slightly in front of the optic foramen. The brain lies 

 loosely in the brain-case, which has no fossae. 



(2.) The Olfactory Chamber. The sphenethmoid appears in the 

 roof of the skull, between and anterior to the two parieto- 

 frontals. Separated by maceration, it is seen to be perforated 

 by two foramina for the olfactory nerves. Posteriorly these 

 foramina open into a common cavity, incomplete above, in which 

 are lodged in the living frog the olfactory lobes of the brain. 

 Anteriorly there are two cavities separated by a mesethmoidal 

 partition. This is carried forwards in the fresh skull by a 

 cartilaginous septum nasi, which forms the partition between 

 the two nasal chambers. These are roofed over by the 

 nasals (na.), somewhat kite-shaped bones, with the thinner tail 

 end directed outwards and backwards. Between these and the 

 pre-maxillse (p. mx.) are the external nares (e. n.). The internal 

 nares (i. n.) are below and somewhat behind them, immediately 

 in front of the palatines. There is no prolonged narial passage 

 as in the rabbit. On the inner side of the internal nares are 



