146 ZOOLOGY sect. 



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

 pelvic. The iiedoral 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 cnim.latory 

 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 nwnih — 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- 

 luccal 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 

 — leads into the large pharynx. Five pairs of slits running 

 vertically on each side of the neck — the braiicliial slits — also 

 lead internally into the pharynx. A large median opening on 

 the ventral surface at the root of the tail, between the pelvic fins, 

 is the opening leading into the cloaca, or "chamber forming the 

 common oiitlet for the intestine and the renal and reproductive 

 organs. A pair of small depressions, the abdominal pores, situated 

 behind the cloacal opening, lead into narrow passages opening into 

 the abdominal cavity. 



The skeleton is composed entirely of cartilage, with, in certain 

 places, depositions of calcareous salts. As in Vertebrates in general, 

 we distinguish two sets of elements in the skeleton — the axial set 

 and the appendicular, the former comprising the skull and spinal 

 column, the latter the limbs and their arches. 



The spinal column is distinguishable into two regions — the 

 region of the trunk and the region of the tail. In the trunk-region 

 each vertebra (Fig. 812, A and B) consists of a centrum (c), neural 

 arch {n.a.), and tran verse processes (tr.pr). In the caudal region 

 there are no transverse processes, but inferior or hccmal arches 

 {G,D, h.a.) take their place. The centra of all the vertebrae are 

 deeply biconcave or amphico&lous, having deep conical concavities 

 on their anterior and posterior surfaces. Through the series of 

 centra runs the notochord (rt^f.), greatly constricted in the centrum 

 itself, and dilated in the large spaces formed by the apposition 

 of the amphicoelous centra of adjoining' vertebra?, where it forms a 

 pulpy mass. The concave anterior and posterior surfaces of the 

 centra are covered by a dense calcified layer, and in Hemiscyllium 

 eight radiating lamelhe of calcified tissue run longitudinally 

 through the substance of the centrum itself. The centra, unlike 



