XTII 



PHYLUM CHOKDATA 



77 



readily removable by boiling or maceration ; or they may eventually 

 become, as it were, grafted on to the cartilage, in which case all 

 distinction between investing and replacing bones is lost in the 

 adult. The investing bones are to be looked upon as portions of 

 the exoskeleton which have retreated from the surface and acquired 

 intimate relations with the endoskeleton. 



The replacing bones have a very definite relation to the regions 

 of tie cartilaginous cranium. In the occipital region four bones 

 are formed, surrounding the foramen magnum : a median ventral 

 basi-occipital (Fig. 774, A and B, B. OC.), paired lateral ex-occipitals 

 (EX. OC.), and a median dorsal supra-occipital (S. OC.). In each 

 auditory "capsule three ossifications commonly appear : a pro-otic 



Fio. 774. A, diagram of bony skull in sagittal section ; B, transverse section of occipital region ; 

 O, of parietal region ; D, of frontal region ; K, of ethmoidal region. Cartilaginous parts are 

 dotted ; replacing bones are marked in thick type, investing bones in italics. M ck. C. MecktTs 

 cartilage ; No. 1 -in, foramina for cerebral nerves ; r. rostrum ; ,. /. sella turcica or pituitary 

 fossa. Replacing bone-; AL. SPH. alisphenoid ; ART. articular ; B.BR. basi-branchial ; 

 B.HY. ba-i-hyal ; B. OC. bast-occipital ; B. SFZZ. basi-sphenoid ; C. BR. cerate-bran- 

 chial ; C.HY. cerato-hyal ; EC.ETH. ecto-ethmoid ; EF.BR. epi-brancliial : EP.HY. 

 epi-hyal : EX.OC. ex-occipital ; II. BR. hvpo-branchial ; H.HY. hypo-hyal ; HY.M. 

 hyomandibular : M. E I H. mesethmoid ; OP.OT. opisthotic ; OR. SPH. orbito-sphe- 

 noid ; PAT., palatine; PH. BR. pharyngo-branchial ; PR. OT. pro-otic; PR. SPH. 

 pre-spheniiiil ; PTOr. pterygoid ; QIT. quadrate ; S. OC. supra-occipital. Investing bones 

 DNT. dentary ; PR. frontal ; MX. maxilla ; NA. nasal ; PA. parietal ; PA. SPH. para- 

 sphenoid ; P-dri". premaxilla ; SQ. sqtiamosa! ; FO., >\n. vomer. 



(A, PR. OT.) in front, an opisthotic (OP. OT.) behind, and an 

 epi-otic (EP. OT.) over the arch of the posterior semicircular canal 

 of the ear (vide infra). In front of the basi-occipital a bone called 

 the basi-sphenoid (A and C, B. SPH.) is formed in the floor of the 

 skull : it appears in the position of the posterior ends of the trabe- 

 culse, and bears on its upper or cranial surface a depression, the sella 

 turcica (s.t), for the reception of the pituitary body. Connected 

 on either side with the basi-sphenoid are paired bones, the 

 all-sphenoids (AL. SPH.), which help to furnish the side- walls of 

 the interorbital region. The basi-sphenoid is continued forwards 

 by another median bone, the pre-fiphenoid (A and D, P. SPH.), 

 with which paired ossifications, the orbito-sphenoids (ORB. SPH), 



