24 Vertebrata. 



elementary characters that is, they display in their 

 permanent state points of structure which other more 

 specialised fishes present only in the embryonic stages. 

 They are for the most part marine, and the skeleton 

 remains almost entirely cartilaginous and never be- 

 comes truly ossified. The notochord, however, rarely 

 persists, being generally replaced by an axis of bicon- 

 cave, often calcified, cartilaginous disks or vertebral 

 bodies. 



The jaws in sharks are usually placed beneath, not 

 at the front of, the prominent snout, and they are both 

 cartilaginous, being, in fact, two parts of a visceral arch. 

 The gills are symmetrical lateral plates of vascular 

 membrane, interposed between pouches which have 

 a row of holes or slits on each side, opening super- 

 ficially on the side of the neck, and internally by a 

 row of perforations into the pharynx. 



The entire surface of the dermis is covered with 

 toothlike papillae, composed of true tooth-tissue or 

 dentine, and these over the jaw arches are large and 

 developed into functional teeth, which are sometimes 

 of formidable size and proportions. There are several 

 rows of these, and as they are gradually worn away 

 with use, they replace each other from within out- 

 wards. This form of dermal scales on the surface of 

 the body is named placoid (fig. 3). 



The vertebral axis is prolonged into the upper 

 lobe of the tail, which thus, on account of the large 

 size of the lower lobe, belongs to the unevenly lobed 

 or heterocercal type, not like the uniform marginal 

 fringe of the lampreys. The fins are often armed 

 with single, strong, and sometimes serrated spines, 

 which are used as weapons of offence. 



