GANOIDEI. 389 



with distinct cranial bones, and the lower jaw is present The 

 exoskeleton is in the form of ganoid scales, plates, or spines. 

 There are usually two pairs of limbs, in the form of fins, each 

 supported by fin-rays. The first rays of the fins are mostly in 

 the form of strong spines. The pectoral arch has a clavicle, and 

 the posterior limbs (ventral fins) are placed close to the anus. 

 The caudal fin is mostly unsymmetrical or " heterocercal." The 

 swim-bladder is always present, is often cellular, and is provided 

 with an air-duct. The intestine is often furnished with a spiral 

 valve. The gills and opercular apparatus are essentially the 

 same as in the Bony fishes. The heart has one auricle and a 

 ventricle, and the base of the branghial artery is dilated into a 

 bulbus arteriosus, which is rhythmically contractile, is furnished 

 with a distinct coat of striated muscular fibres, and is provided 

 with several transverse rows of valves. 



Of these characters, the ones which it is most important to 

 remember are the following :— 



I. The endoskeMon 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 de- 

 veloped, and the skull is cartilaginous, and is protected by 

 ganoid plates. Even in these forms, however, the peripheral 

 elements of the vertebrae are ossified. In others, the bodies of 

 the vertebrae are marked out by osseous or semi-cartilaginous 

 rings, enclosing the primitive matter of the iiotochord. In 

 others, the vertebrae are like those of the Bony fishes — that is 

 to say, deeply biconcave or " amphiccelous." In one Ganoid, 

 however — the Bony Pike (Lepidosteus) — the vertebral column 

 consists of a series of " opisthoccelous " 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 stmcture is more Reptilian than 

 Piscine. 



II. The exoskeleton consists in all Ganoid fishes 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 overlapping, in oblique rows, the plates 

 of each row being often articulated to those of the next by dis- 

 tinct processes (fig. 132, </). In other cases the ganoid struc- 

 tures are simply in the form of detached plates, tubercles, or 



