io6 A HISTORY OF FISHES 



Except in the Lampreys and their allies, the mouth of a fish 

 is always supported internally by structures known as jaws. 

 In order to understand the origin of these jaws it is necessary 

 to consider again the half-hoops of cartilage or branchial 

 arches, which in the Selachians lie in the side walls of the 

 pharynx between the internal openings [cf. p. 35). There is 

 little doubt that the earliest fishes possessed a series of these 

 arches, all of them connected with gills, and that after a time 

 the first two pairs became specially modified (Fig. 46A). The 

 first pair was transformed into biting jaws, consisting of an 

 upper portion known as the pterygo-quadrate cartilage {ptq.), 

 and a lower portion known as Meckel's cartilage (mk.). In some 

 Sharks living to-day this first or mandibular arch still exhibits 

 traces of its original character, and may lie in front of a gill- 

 cleft and be associated with vestigial gills; in the remainder it 

 has lost all trace of its branchial origin, and only the manner 

 of its development provides a clue as to how it came into being. 

 The second or hyoid arch has been much less modified, and is 

 not very unlike the branchial arches which lie behind it. Its 

 normal function is to provide a support for the tongue, but in 

 most fishes it has acquired the secondary task of suspending the 

 mandibular arch from the cranium (Fig. 46A; hym.). 



Examination of the skull of the Spotted Dog-fish [Scyliorhinus) 

 shows that each upper jaw is connected with its fellow in front 

 below the cranium, and that the two halves of the lower jaw 

 are similarly bound together. Further, the upper jaw is attached 

 to the cranium by a muscular ligament at about the middle of 

 its length, and the hinder ends of both jaws are slung from the 

 back part of the cranium by the intervention of one of the seg- 

 ments of the second arch, namely, the hyomandibular cartilage 

 (Fig. 46A). In the Comb-toothed Sharks {Hexanchidae) the 

 mode of suspension is somewhat dififerent, the upper jaw being 

 not only joined to the cranium by a process at the middle of 

 its length, but also has another direct articulation with that 

 part of the cranium which lies behind the eye-socket or orbit. 

 Being relieved from taking part in the suspension of the jaws, 

 the hyomandibular cartilage is here reduced to a relatively 

 slender rod, and below is connected with the remainder of the 

 hyoid arch. Yet another type of suspension is found in the 

 Bull-headod Sharks [Heterodontidae) ; the jaws are slung from the 

 cranium by the hyomandibular, but the upper jaw fits into a 

 deep groove in the cranium and is firmly attached to it by 

 strong ligament. In the Chimaeras [Holocephali) this condition 



