72 ZOOLOGY SECT. 



In connection with and always superficial to the rostrum, 

 olfactory capsules, and jaws are frequently found labial cartilages 

 (Ib. 1-4), which sometimes attain a great size. 



In the lower Fishes, such as Elasmobranchs, the cartilages of 

 the skull become more or less encrusted by a superficial granular 

 deposit of lime-salts, giving rise, as in the vertebral column, to 

 calcified cartilage, but in all the higher forms true ossification 

 takes place, the cartilaginous skull becoming complicated, and 

 to a greater or less extent replaced, by distinct bones. Of these 

 there are two kinds, cartilage and membrane bones. Cartilage 

 bones begin by the deposition of minute patches of calcific matter 

 in the cartilage itself: these centres of ossification are not dis- 

 tributed irregularly, but have definite positions, constant in 

 the whole series of higher Craniata. As development proceeds, 

 they may be replaced by ossification, starting in the perichondrium, 

 or layer of connective tissue surrounding the cartilage, and gradu- 

 ally invading the latter. But in either case the bones in question 

 are preformed in cartilage, i.e. replace originally cartilaginous 

 parts. In the case of membrane bones centres of ossification 

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

 cartilage : they may remain quite independent of the original 

 cartilaginous skull and its cartilage 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 membrane and cartilage bones is lost in 

 the adult. The membrane 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 cartilage 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 

 basi-occipital (Fig. 720, 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 opistJiotic (OP. OT) behind, and an epiotic 

 (EP. OT) over the arch of the posterior semicircular canal (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 on each side with the basi-sphenoid are paired bones, the 

 alisphenoids (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-splunoid (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. 



