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PHYLUM CHORDATA 145 



complete hoops of cartilage, mostly segmented, which lie in the 

 sides and floor of the mouth-cavity or pharynx. The first of these 

 forms the upper and lower jaws. The upper jaw, or palatoquadrate 

 (pal. q.), consists of two stout rods of cartilage firmly bound to- 

 gether in the middle line and bearing the upper series of teeth. 

 The lower jaw, or Meckel's cartilage (mck.), likewise consists of two 

 stout tooth-bearing cartilaginous rods firmly united together in the 

 middle line, the union being termed the syrnphysis. At their outer 

 ends the upper and lower jaws articulate with one another by a 

 movable joint. In front the upper jaw is connected by a ligament 

 with the base of the skull. 



Immediately behind the lower jaw is the Jiyoid arch. This 

 consists of two cartilages on each side, and a mesial one below. 

 The uppermost cartilage is the hyomandibular (hy. mn) : this 

 articulates by its proximal end with a distinct articular facet 

 on the auditory region of the skull : distally it is connected by 

 ligamentous fibres with the outer ends of the palatoquadrate 

 and Meckel's cartilage. The lower lateral cartilage is the cerato- 

 hyal (cer. hy.}. Both the hyomandibular and ceratohyal bear a 

 number of slender cartilaginous rods the branchial rays of the 

 hyoid arch. The mesial element, or basihyal (b.hy.), lies in the 

 floor of the pharynx. Behind the hyoid arch follow the branchial 

 arches, which are five in number. Each branchial arch, with 

 exceptions to be presently noted, consists of four cartilages. 

 The uppermost of these pharyngobranchial (ph. br.l-ph. br.ij] lie 

 in the dorsal wall of the pharynx, not far from the spinal column ; 

 the pharyngobranchials of the last two arches are fused together. 

 The next in order the epibranchials (ep. br.) with the exception 

 of those of the last arch, bear a number of slender cartilaginous 

 rods the branchial rays which support the walls of the gill-sacs ; 

 and the next the ceratobranchials (cer. br.) are, with the same 

 exception, similarly provided. The hypobranchials (hyp. br.), which 

 succeed these, are absent in the case of the first and fifth arches. 

 In the middle line on the floor of the pharyngeal cavity is a mesial 

 cartilage the basibranchial (Fig. 824, b. br.) which is connected 

 with the ventral ends of the third, fourth, and fifth arches. A 

 series of slender curved rods the extrabranchials lie superficial 

 to the branchial arches, along the borders of the corresponding 

 external branchial clefts. 



Two pairs of delicate labial cartilages are present at the sides 

 of the mouth, and a couple at the margins of the openings of the 

 olfactory capsules. 



The skeleton of all the fins paired and unpaired presents a 

 considerable degree of uniformity. The main part of the expanse 

 of the fin is supported by a series of flattened segmented rods, the 

 pterygiophores or cartilaginous fin-rays, which lie in close apposition : 

 in the case of the dorsal fins these may be partly calcified. At 



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