212 THE DOG-FISH 



2. The second or hyoidean arch is incomplete above : it 



consists of a pair of rods the dorsal ends of which 

 articulate with the sides of the skull, while the ventral 

 ends are connected together by a median plate of 

 cartUage. Each rod is further divided into an upper 

 and a lower segment. 



a. The hyo-mandibular cartilage, or upper segment, is 



a short stout bar of cartilage, projecting almost 

 horizontally outwards from the skull, but slightly 

 downwards and backwards. Its inner and upper 

 end articulates with a concave surface on the side 

 of the skull, near its hinder end, and immediately 

 below the post-orbital groove : its outer end arti- 

 culates with the lower segment of the arch, oppo- 

 site the angle of the mouth. To its hinder border 

 are attached a series of rods of cartilage — ^the 

 gill-rays — which support the gill-folds. 



The hyo-mandibular cartilage in the dog-fish 

 is spoken of as the suspensorium, because it forms 

 the skeletal Hnk between the jaws and the skuU. 



Skulls, like that of the dog-fish, in which the 

 hyoidean arch forms the suspensorium are spoken 

 of as hyostylic. 



b. The cerato-hyal, or lower segment of the hyoidean 



arch, is a longer and more slender bar which runs 

 forwards and inwards in the floor of the mouth, 

 just behind the lower jaw, and partly overlapped 

 by it. It bears gill-rays along its posterior border. 



c. The basi-hyal is a broad median plate of cartilage, 



lying in the floor of the mouth behind the lower 

 jaw. It is rounded anteriorly, and produced 

 behind into two horns which are attached by 

 ligaments to the dorsal surfaces of the cerato- 

 hyals. 



3. The branchial arches are the remaining five visceral 



arches : they diminish in size from before backwards, 

 and each is divided into four segments on each side. 



