78 ZOOT.OriY SKCT. 



posterior margin of the month and nnito in the middle line, 

 forming the loicer jaw. The qitadrate {qu.), or posterior end of 

 the palato-quadrate, furnishes an articulation for the lower jaw, 

 and often acquires a connection with the cranium, thus serving 

 10 suspend the jaws from the latter. Thus each jaw arises from 

 the union of paired bars, the final result being two unpaired 

 transverse structures, one lying in the anterior, the other in the 

 posterior margin of the transversely elongated mouth, and moving in 

 a vertical plane. The fundamental difference between the jaws 

 of a Vertebrate and the structures called by the same name in an 

 Arthropod or a Polychaitous Worm will be obvious at once. 



The hyoid bar usually becomes divided into two parts, a dorsal, 

 the hyoniandibular or lyharyngo-hi/al (hy.m.), and a ventral, the hyoid 

 cornu, which is again divisible from above downwards into segnients 

 called resipectively epi-hyal {e^Djiy), cerato-hyal {c.hy.), and hypo-hyal 

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

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

 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 suspensoriura or jaw-suspending appara- 

 tus — a mandihular suspensorium, furnished by the quadrate, and a 

 hyoidean snsp)ensorium formed b}" the hyoniandibular : in the former 

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

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

 instances an ampliistylic arrangement is produced by the presence 

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

 auditory region of the skull, and an indirect connection through 

 the hyoniandibular. 



The branchial arches become divided transversely into dorso- 

 ventral segments called respectively pharyngo-hranchial {2^h. hr.) 

 epi-hranchial {epi.hr.), ccrcdo-hrancJiial (chr.), and hypo-hranchial 

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

 an articulated framework Avhich 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 lahial cartilaycs 

 (lb. 1 — ^), which sometimes attain considerable dimensions. 



In certain 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 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 hones. 

 Of these there are two kinds, replaciiig or " cartilage " — and investing 

 or " membrane " — hones. Replacing bones may begin by the de- 

 position of patches of bony matter in the cartilage itself {endo- 

 chondral ossification). As development proceeds, these may be 



