xiii PHYLUM CHORDATA 79 



replaced by ossification starting .within the pertchcndrium , or layer 

 of connective-tissue surrounding the cartilage, and gradually 

 invading the latter. More usually the bone is formed from the 

 outset by the deposition of layers invading the cartilage from the 

 perichondrium (or periosteum) inwards (perichondral or periosteal 

 ossification). But in either case the bones in question are usually 

 said to be preformed in cartilage, i.e., they replace originally 

 cartilaginous parts. In the case of investing bones centres of 

 ossification also appear, in constant positions, in the fibrous tissue 

 outside the cartilage : they may remain quite independent of the 

 original cartilaginous skull and its replacing bones, so as to be 

 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 the cartilaginous cranium. In the occipital region four bones 

 are formed, surrounding the foramen magnum : a median ventral 

 la si-occipital (Fig. 765, 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 

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

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

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

 nected oiveach 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 prc-sphcnoid (A and D, P. SPH.), with 

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

 connected, and complete the side walls of the interorbital region. 

 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 replacing bones furnish floor and side- walls only. This 

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

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

 and usually articulating below with the ali-sphenoids, and the 

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



