PHYLUM CHORDATA 



269 



cipital : it presents on its outer surface, behind the otic process of 

 the suspensorium, a small aperture, the fenestra ovalis, closed in the 

 entire animal by membrane, and, when the latter is removed, 

 leading into the cavity of the auditory capsule, containing the 

 membranous labyrinth. 



In front of the auditory capsules a considerable part of the 

 cranial wall is formed of cartilage, and presents above a single 

 large and a pair of small fontanelles (Fig. 922, fon., fon') but 

 anteriorly it is ossified by the sphenethmoid, or girdle-bone 

 (SP. ETH), a short bony tube divided by a transverse partition 

 into an anterior compartment which lodges the hinder ends of 



J^zi.s 



Nv. s, 7 



ez-cTvy 



itjuac{] 



QU-jr. 



Nv. q, ^o 



ivw. 9 to. 



Fig. 923. — Sana temporaria. The skull. A, from beneath, with the investing bones 

 removed on the right side (left of figure) ; B, from the left side, with mandible and hyoid ; 

 C, from behind, the investing bones removed at sws. a. c. liy. anterior cornu of hyoid; 

 awd. rj>. auditory capsule ; &. hy. body of hyoid ; COIi. columella ; T>NT. dentary ;EX.OC . 

 exoccipital ; /or. taag. foramen magniun ; FR.PA. fronto-parietal ; lA.zaCK. mento- 

 meckelian ; MX. maxilla ; NA. nasal ; Nv. S, optic foramen ; Nv. 5, 7, foramen for fifth 

 and seventh nerves ; Nv. 9, 10, foramina for ninth and tenth nerves ; oc. en. occipital 

 condyle ; otf. cp. olfactory capsule ; ot, pr. otic process ; PAL. palatine ; pal. qu. palato. 

 quadrate; 'PA.SPff. parasphenoid; p. c. hy. posterior cornu of hyoid; ped. pedicle; 

 PAfX premaxilla; PR. OT. pro-otic; iTS. pterygoid ; Cf^./K quadrato-jugal; SF.ETH. 

 sphenethmoid ; SQ, paraquadrate ; stp. stapes ; sus (quad) suspensorium (quadrate) ; VO. 

 vomer. (After Howes, slightly altered.) A minute investing bone, the septo-^maxiUary, 

 which is present above the maxilla, close to the nostril, is not here represented. 



the olfactory sacs, and a posterior compartment which contains 

 the olfactory bulbs. The anterior compartment is again divided 

 by a vertical partition which separates the olfactory sacs from one 

 another, and the transverse partition is perforated for the olfac- 

 tory nerves. This very peculiar and characteristic bone may be 

 taken to represent meso- and ecto-ethmoids and pre- and orbito- 

 sphenoids all united together. 



The olfactory capsules (Figs. 922, 923, olf. cp) have a delicate 

 cartilaginous roof and floor produced into irregular processes which 

 help to support the olfactory sac. They are separated from one 

 another by a vertical plate of cartilage, continuous behind with the 



