xm PHYLUM CHORDATA 141 



As in Fishes in general, two sets of fins are to be recognised the 

 unpaired or median fins, and the paired or lateral. These are all 

 flap-like outgrowths, running vertically and longitudinally in the 

 case of the median fins, nearly horizontally in the case of the lateral : 

 they are flexible, but stiffish, particularly towards the base, owing 

 to the presence of a supporting framework of cartilage. Of the 

 median fins two the dorsal are situated, as the name indicates, 

 on the dorsal surface : they are of triangular shape ; the anterior, 

 which is the larger, is situated at about the middle of the length of 

 the body, the other a little further back. The caudal fin fringes the 

 tail : it consists of a narrower dorsal portion and a broader ventral, 

 continuous with one another round the extremity of the tail, the 

 latter divided by a notch into a larger, anterior, and a smaller, 

 posterior lobe. The tail is heterocercal, i.e., the posterior extremity 

 of the spinal column is bent upwards and lies in the dorsal portion 

 of the caudal fin. The ventral or so-called analjm is situated on 

 the ventral surface, in Scyllium opposite^the' interval between* the 

 anterior and posterior dorsals, in Hemiscyllium behind the latter ; 

 it resembles the -latter in size and shape. 



FIG. 821. Dog-Fish (Hemiscyllium modestum). Lateral view. (After Waite.) 



Of the lateral fins there are two pairs, the pectoral and the pelvic. 

 The pectoral are situated at the sides of the body, just behind 

 the head. The pelvic, which are the smaller, are placed on the 

 ventral surface, close together, about the middle of the body. 

 In the males the bases of the pelvic fins are united together in the 

 middle line, and each has connected with it a clasper or copulatory 

 organ. The latter is a stiff rod, on the inner and dorsal aspect 

 of which is a groove leading forwards into a pouch-like depression 

 in the base of the fin. 



The mouth a transverse, somewhat crescentic opening is 

 situated on the ventral surface of the head, near its anterior end. 

 In front and behind it is bounded by the upper and lower jaws, 

 each bearing several rows of teeth with sharp points directed back- 

 wards. The nostrils are situated one in front of each angle of the 

 mouth, with which each is connected by a wide groove the naso- 

 buccal groove. In Hemiscyllium the outer edge of the groove is 

 prolonged into a narrow subcylindrical appendage the barbel. A 

 small rounded aperture, the spiracle, placed just behind the eye 



