250 



ZOOLOGY 



SECT. 



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. 870, fon. fon'} but 

 anteriorly it is ossified by the sphen-etkmoid, or girdle-bone 

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

 into an anterior compartment which lodges the hinder ends of 

 the olfactory sacs, and a posterior compartment which contains 

 the olfactory lobes. The anterior compartment is again divided 

 by a vertical partition which separates the olfactory sacs from one 



EX.OC 



FIG. 871. Rana temporaria. The skull. A, from beneath, with the membrane bones re- 

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

 C, from behind, a. c. h>/. anterior cornu of hyoid ; b. )iy. body of hyoid ; COL. columella ; 

 DNT. dentary ; EX.OC. ex-occipital ; for. mag. foramen magnum ; FR. PA. fronto-parietal ; 

 M.MCK. meiito-meckelian ; MX. maxilla; NA. nasal; Ni: 2, optic foramen; Nv. 5, 7, fora- 

 men for fifth and seventh nerves ; oc. en. occipital condyle ; olf. rp. olfactory capsule ; of. }>r. 

 otic process ; PAL. palatine ; >/. qu. palato-quadrate ; P^.-SP/f-'parasphenoid ; p. c. In/, pos- 

 terior cornu of hyoid ; ped. pedicle ; PMX. premaxilla ; PR.OT. pro-otic ; PTG. pterygoid ; 

 QU.JU. quadrato-jugal ; SP.ETH. sphenethmoid ; SQ. squamosal ; sfj>. stapes; VO. vomer. 

 (After Howes, slightly altered.) 



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 (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 Avail of each is produced into a little curved, rod-like 

 I'liinal iirocess. The whole of the palato-quadrate arch is un- 

 ossified. 



To this partly ossified chondrocranium the usual membrane 



