EXTERNAL FEATURES. 119 



fathoms). Their flesh is not usually esteemed as food, but some 

 of them are eaten by poor people. 



The body is elongated and spindle-shaped in the Squall^. the 

 anterior part being somewhat broad and depressed dorso-ven- 

 trally as compared with the narrower posterior region ; in the 

 Raji it is strongly compressed dorso-ventrally. In some forms, 

 mostly in the Raji, the snout is prolonged to a greater or less 

 extent. This is most markedly the case in the saw-fish shark 

 and in the saw-fish Pristis. In the hammer- 



heads the anterior part of the head is_elongatedjtransversely, 

 the eyes being placed at the ends of the prolongations. The\ 

 median fins are typically two dorsal, a _caudal, the ventral 

 part of which is divided by a notch into two parts, and^an^anal/ 

 placed between the caudal and the anus. The paired fins are, 

 well-developed : the pelvic being smaller than the pectoral,^ 



FIG. 64. Acantffl&s vulgaris. spl spiracle ; ks gill slits (from Claus). 



and abdominal in position. In the males the pelvic fins are 

 provided, each with a copulatory appendage the clasper 

 (pterygopodmm, mixipterygium), which is grooved on its dorsal 

 side, the groove leading into a cavity at the base of the appendage. 

 In the Raji the pectoral fins are very large and their line of 

 attachment to the body has a considerable antero-posterior 

 extension. The muscular system is on the usual piscine type. 

 The great lateral muscle is divided into a dorsal and ventral half, 

 the myomeres of which alternate. There is the usual system 

 of branchial and mandibular muscles. 



The gill-clefts are tubes usually five on each side (in Chlamy- 

 doselachus and Hexanchus there are six, in Heptanchus seven), 

 and their external openings which are placed laterally in Squall, 

 ventrally in Raji, are not covered by an operculum (Fig. 64). 

 Internally they open into the pharynx and their walls are pro- 



