2S6 AN INTRODUCTION TO ZOOLOGY. 



gular shape, and sometimes armed with a projecting 

 spine. 



The Ganoidei, fishes which are not so numerous 

 now as in former ages, receive their name from the 

 polished armour of scales with which many kinds are 

 invested. These scales are covered with a layer of 

 polished enamel, derived from the epidermis; they are 

 fitted into one another, but scarcely overlap. The best 

 known living member of the order is the Sturgeon : it 

 has a cartilaginous skeleton and a persistent notochord. 

 But the cartilaginous skull is supplemented by what 

 are called dermal bones, and in many other ganoid 

 fishes there are a number of well-developed bones. 

 The group is therefore in a manner intermediate be- 

 tween the Dog-fishes and the Bony Fishes, although, as 

 regards its armour of scales, it is unlike either. The 

 scales themselves, with their polished enamel, may be 

 compared to teeth placed outside ; but although they 

 have been distinguished by a special name, owing to 

 their coat of enamel, it must be remembered that they 

 are not greatly different from the so-called placoid 

 scales, which also are covered by an element derived 

 from the epidermis, though it is not so noticeable. 



The Bony Fishes, or Teleostei, constitute the great 

 majority of fish, including all the familiar kinds, such 

 as the herring, eel, pike, salmon, cod, mackerel, etc. 

 Their skeleton is bony, not cartilaginous. The noto- 

 chord persists as a soft tissue placed centrally be- 

 tween the joints of the backbone. Bach joint of bone 

 presents a hollow on both sides, where a soft pad of 



