XIII 



PHYLUM CHORDATA 



3 



The basi-occipital, basi-sphenoid, and pre-sphenoid together form 

 the basis cranii of the bony skull. A vertical plate of bone, the 

 mesethmoid (M. ETH.), appears in the posterior portion of the car- 

 tilage of the same name, and the outer walls of the olfactory 

 capsules may be ossified by paired ecto-ethmoids (E, EC. ETH). 



So far, it will be seen, the cranial cavity has its hinder region 

 alone roofed over by bone, viz. by the supra-occipital : for the rest 

 of it the cartilage bones furnish floor and side walls only. This 

 deficiency is made good by two pairs of membrane bones, the 

 parietals (PA), formed immediately in front of the supra-occipital, 

 and usually articulating below with the alisphenoids, and the 



R SPH 



M.ETH 



HHY 8.BR 



FIG. 720. Diagram of bony skull in sagittal section ; B, transverse section of occipital region ; 

 C, of parietal region ; D, of frontal region ; E, of ethmoidal region. Cartilaginous parts are 

 dotted ; cartilage bones are marked in thick type, membrane bones in italics ; nick. c. Meckel's 

 cartilage ; i\>. 1 10, foramina for cerebral nerves ; /. rostrum ; 5. t. sella turcica or pituitary 

 fossa. Cartilage bones AL.SPH . alisphenoid ; ART. articular : B. BR. basi-branchial ; 

 B. HY. basi-hyal ; B. OC.- basi-occipital ; B. SPH. basi-sphenoid; C. BR. cerato-bran- 

 chial ; C. HY. cerato-hyal ; EC. ETH. ecto-ethmoicl ; EP. BR. epi-branchial ; EP. H Y. 

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

 hyomandibular ; M, ETH. mesethmoid; OP.OT. opisthotic ; OR. SPH'. orbito-sphe- 

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

 pre-sphenoid; PTG. ptervgoid ; QU. quadrate; 8. OC. supra-occipital. Membrane bones 

 DNT. dentary; FR. frontal ; 3IX. maxilla; XA. nasal; PA. parietal; PA.* PH. parasphe- 

 noid ; PM.X. prernaxilla ; Sty. squamosal ; VO. vomer. 



frontals (FR), placed in front of the parietals, and often connected 

 below with the orbito-sphenoids. A pair of nasals (JV14) are 

 developed above the olfactory capsules and immediately in advance 

 of the frontals ; and below the base of the skull two important 

 membrane bones make their appearance, the vomer ( VO) which 

 may be double in front, and the paraspJienoid (PA. SPH) 

 behind. 



The result of the peculiar arrangement of cartilage and mem- 

 brane bones just described is that the brain-case, in becoming 

 ossified, acquires a kind of secondary segmentation, being clearly 

 divisible in the higher groups, and especially in the Mammalia, 

 into three quasi-segments. These are the occipital segment 



