198 ZOOLOGY SECT. 



produced downwards into paired longitudinal ridges, enclosing 

 between them a groove which is converted into a canal by the 

 apposition of the parasphenoid bone and serves for the origin of the 

 eye-muscles. In front of the auditory region the cranium is exca- 

 vated on each side by a large orbit, a vertical plate or inter -orbital 

 septum (OR. SPH.) separating the two cavities from one another. 

 In front of the orbital region the cranium broadens out to form 

 the olfactory capsules, each excavated by a deep pit (olf. s.) for the 

 olfactory sac, and anterior to these is a blunt snout or rostrum. 

 The occipital region is formed as usual from the parachordals of 

 the embryonic skull, the auditory region from the auditory capsules, 

 and the rest of the cranium from the trabecula3. 



The replacing bones, formed as ossifications in the chondro- 

 cranium, correspond in essentials with the typical arrangement 

 already described (p. 77). In the occipital region are four bones ; 

 the basi-occipital (B. OC.), forming the greater part of the occipital 

 condyle and the hinder region of the basis cranii or skull-floor : 

 the ex-occipitals (EX. OC.), placed one on each side of the foramen 

 magnum and meeting both above and below it ; and the supra- 

 occipital (S. OC.), forming the occipital crest already noticed. 

 Each auditory capsule is ossified by five bones i.e., two more than 

 the typical number (p. 77) : the pro-otic (PR. OT.), in the anterior 

 region of the capsule, uniting with its fellow of the opposite side 

 in the floor of the brain-case, just in front of the basi-occipital ; 

 the opisihotic, in the posterior part of the capsule, external to the 

 ex-occipital ; the sphenotic (SPH. OT.), above the pro-otic and 

 forming part of the boundary of the orbit ; the pterotic (PT. OT.), 

 above the ex-occipital and opisthotic, forming a distinct lateral 

 ridge and produced behind into a prominent pterotic process ; and 

 the epiotic (EP. OT.), a small bone, wedged in between the supra- 

 and ex-occipitals and pterotic, and produced into a short epiotic 

 process. On the external face of the auditory capsule, at the 

 junction of the pro-, sphen-, and pter-otics, is an elongated facet 

 (h.m.) covered with cartilage and serving for the articulation of 

 the hyomandibular. 



The trabecular region of the cranium contains six bones. Imme- 

 diately in front of the conjoined pro-otics, and forming the anterior 

 end of the basis jcranii, is a small unpaired Y-shaped bone, the 

 basisphenoid (B. SPH.). Above it, and forming the anterior 

 parts of the side-walls of the brain-case, are the large paired 

 alisphenoids (AL. SPH.). In the inter-orbital septum is a median 

 vertical bone, representing fused orbitosphenoids (OR. SPH.). 

 Lastly, in the posterior region of each olfactory capsule, and 

 forming part of the boundary of the orbit, is the ecto-eihmoid 

 (EC. ETH.). 



The investing bones already referred to are closely applied to 

 the roof and floor of the chondrocranium, and modify its form 



