40 ORDER GANOIDEI; GENERAL CHARACTERS. 



planet, the members of this order constituted almost the only re- 

 presentatives of the class of Fishes in its waters ; and their fossil 

 remains, preserved in the older strata of the earth, show the 

 abundance and astonishing variety of forms which must formerly 

 have existed in the waters of the ancient world. Some remarks 

 upon these will be found in the paragraphs devoted to fossil 

 Fishes, at the end of this chapter. 



593. The character upon which the order of Ganoid Fishes 

 was originally founded, and that to which its name refers, 

 consists in the nature of the covering of the skin in most of its 

 members ; this is composed usually of bony scales or plates, coated 

 with a thin layer of enamel, so that the Fishes are, as it were, 

 encased in a suit of bony armour. A somewhat similar structure, 

 however, occurs, as we shall see, in many Fishes belonging to 

 the following order; and these were accordingly included by 

 Professor Agassiz, whose principal object was the facilitation of 

 the study of fossil Fish, in the order of Ganoidei, as originally 

 proposed by him. The primary characters adopted by Professor 

 Miiller for the distinction of the order, as he circumscribed it, 

 consist, as already stated, in the muscular coating of the bulb of 

 the aorta, and the presence of valves in its interior, and in the 

 existence of a spiral valve in the intestine ; whilst these Fishes 

 are distinguished from the Selachii, in which these characters 

 also occur, by their free branchiae and movable opercula. 



594. In other respects the Ganoid Fishes exhibit a gradual 

 transition from forms very nearly allied to the Selachii, to 

 others which exhibit perhaps a still closer relationship to the 

 true bony Fishes. The skeleton is sometimes cartilaginous, the 

 vertebral column being even represented in some species by a 

 gelatinous dorsal chord, and sometimes perfectly ossified; the 

 skin is sometimes quite naked, sometimes covered with the bony 

 plates above mentioned, and sometimes with horny scales with or 

 without a coating of enamel ; but in most cases the head, at all 

 events, is covered with enamelled plates of bone. The tail is 

 sometimes heterocercal, sometimes symmetrical ; and the fin-rays 

 are sometimes soft and flexible, sometimes divided like the soft 

 rays of the fins of bony Fishes, and sometimes spinous. Each 



