PHYLUM CHORDATA 



261 



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. 933, /on., fori), 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 com- 

 partment which lodges the hinder ends of the olfactory sacs, and a 

 posterior compartment which contains the olfactory bulbs. The 



/MX 



olf. 



>. EX.OC 

 p'erf. 



FIG. 934. Rana temper aria. 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 sus. a. c. hy. anterior cornu of hyoid ; 

 aud.cp. auditory capsule ; b. hy. body of hyoid ; COL. columella ; DNT. dentary ; EX.OC. 

 exoccipital ; for. mag. foramen magnum ; /. ov. fenestra ovalis ; FR.PA. fronto-parietal ; 

 M.1KCK. mento-meckelian : MX. maxilla ; NA. nasal ; Nv. 2, optic foramen ; Nv. 5, 7, 

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

 occipital condyle ; olf. cp. olfactory capsule ; ot. pr. otic process ; PAL. palatine ; pal. qu. 

 palato-quadrate ; PA.SPH. parasphenoid ; p. c. hy. posterior cornu of hyoid ; ped. pedicle ; 

 PMX. premaxilla ; PR.OT. pro-otic ; PTG. pterygoid ; Q U.JU. quadra to- jugal ; SP.ETH. 

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

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

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



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 olfactory 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. 933, 934, 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 

 girdle-bone and representing the unossified part of the mesethmoid ; 

 and the anterior wall of each is produced into a little curved, rod- 



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