70 ZOOLOGY 



SECT. 



the hyomandibular or pJiaryngo-hyal (hy.m.), and a ventral, the hyoid 

 cornti, which is again divisible from above downwards into segments 

 called respectively epi-hyal (ep.hy.), cerato-hyal (c.hy.), and hypo-hyal 

 (h.hy.). The median ventral element of the arch, or basi-hyal (b.hy.), 

 serves for the support of the tongue. In some Fishes the hyoman- 

 dibular articulates above with the auditory region of the cranium, 

 while the jaws are connected with its ventral end. We may thus 

 distinguish two kinds of suspensorium or jaw-suspending appara- 

 tus a mandibular suspensorium, furnished by the quadrate, and a 

 hyoidean suspensorium, formed by the hyomandibular : in the former 

 case the skull is said to be autostylic, i.e., having the jaw connected 

 by means of its own arch ; in the latter it is called hyostylic : in a few 

 instances an amphistylic arrangement is produced by the presence 

 both of a direct articulation between the palato-quadrate and the 

 posterior region of the skull, and an indirect connection through 

 the hyomandibular. 



The branchial arches become divided transversely into dorso- 

 ventral segments called respectively pharyngo-branchial (ph. br.), 

 epi-branchial (ep.br.), cerato-branchial (c.br.), and hypo-branchial 

 (h.br.), and the visceral skeleton thus acquires the character of 

 an articulated framework which allows of the dilatation of the 

 pharynx during swallowing and of its more or less complete closure 

 at other times. 



In connection with, and always superficial to. the rostrum, 

 olfactory capsules, and jaws, are frequently found labial cartilages 

 (Ib. 1 4), which sometimes attain considerable dimensions. 



In certain Fishes, such as Elasmobranchs, the cartilages of 

 the skull become more or less encrusted by a supsrficial granular 

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

 these Fishes, to calcified, cartilage ; but in all the higher forms true 

 ossification takes place, the cartilaginous skull becoming compli- 

 cated, and to a greater or less extent replaced, by distinct bones. 

 Of these there are two kinds, replacing or " cartilage "-bones and in- 

 vesting or " membrane "-bones. Replacing bones may begin by the 

 deposition of patches of bony matter in the cartilage itself (endo- 

 chondral ossification). As development proceeds, these may be 

 replaced by ossification starting within the perichondrium, 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 



