32O ORDERS OF FISHES. 



skeleton is only partially ossified, the vertebral column mostly re- 

 maining cartilaginous throughout life, especially amongst the extinct 

 forms of the Paleeozoic period, in which the notochord is persistent. 

 The skull is furnished 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 pro- 

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

 spiral valve. T/ie 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 branchial 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, those which it is most important to 

 remember are the following : 



I. The endoskeleton 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 notochord. In 

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

 to say, deeply biconcave or " amphicoelous." 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 structure 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 



