ORDERS OF FISHES. 131 



tile, is furnished ivith a distinct coat of striated muscular fibres, 

 and is provided with several transverse rows of vcdves. 



Of these characters, those which it is most important to 

 remember are the following : — 



1. The endosl-celeton is rarely thoroughly ossified, but varies 

 a good deal as to the extent to which ossification is carried. 

 In some forms, including most of the older members of the 

 order, the chorda dorsalis is persistent, no vertebral centra 

 are developed, and the skull is cartilaginous, and is protected 

 by ganoid plates. Even in these forms, however, the peri- 

 pheral elements of the vertebrae may be ossified. In others, 

 the bodies of the vertebrae are marked out by osseous or 

 semi-cartilaginous rings, enclosing the primitive matter of the 

 notochord. In others, the vertebrae are like those of the 

 Bony fishes — that is to say, deeply biconcave or " amphi- 

 ccelous." In one Ganoid, however — the Bony Pike {Lepi- 

 dosteus) — the vertebral column consists of a series of " opis- 

 thocoelous " vertebrae — that is to say, vertebrae which are 

 convex in front and concave behind. This is the highest 

 point of development reached in the spinal column of any 

 fish, and its structure is more Eeptilian than Piscine. 



2. The exoskcleton consists, in all Ganoid fishes in which 

 it is present, of scales, plates, or spines, which are said to 

 possess ganoid characters. The peculiarities of these scales 

 are that they are composed of two distinct layers — an inferior 

 layer of bone and a superficial covering of a kind of enamel, 

 somewhat similar to the enamel of the teeth, called " ganoine." 

 In form the ganoid scales most generally exhibit themselves 

 as rhomboidal plates, placed edge to edge, without overlap- 

 ping, in oblique rows (fig. 499), the plates of each row being 

 often articulated to those of the next by distinct processes. 

 In other cases the ganoid structures are simply in the form 

 of detached plates, tubercles, or spines ; and in some cases 

 their shape is undistinguishable from the horny scales of the 

 typical Teleostean fishes, being circular and overlapping. It 

 is to be remembered, however, that these ganoid plates and 

 scales are not confined to the fishes of the order Ganoidei, 

 but that they occur in two sub-orders of the Bony fishes 

 — namely, the Plcctognathi and Lojfliobranchii — and in some 



