VERTEBRATA : FISHES. 489 



CHAPTER LIV. 

 GANOIDEL 



ORDER IV. GANOIDEL The fourth order of fishes is the large 

 and important one of the Ganoid fishes, represented, it is true, 

 by few living forms, but having an enormous development in 

 past geological epochs. For this reason the study of the 

 Ganoid fishes is one which claims considerable attention. 



At the present day, the order Ganoidei comprises only seven 

 genera viz., Lepidosteus, Polypterus, Calamoichthys, Amia, 

 Acipenser, Scaphirhynchus, and Spatularia all of which are 

 found only in the northern hemisphere, and are wholly or 

 partially confined to fresh water. 



The order Ganoidei may be defined by the following charac- 

 ters : The endoskeleton is only partially ossified, the vertebral 

 column mostly remaining cartilaginous throughout life, especially 

 amongst the extinct forms of the Palceozoic period, in which the 

 notochord is often 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 tm- 

 sym metrical or ' ' heterocercal. " The swim-bladder is always pres- 

 ent, 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 

 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 ivith several transverse 

 rows of valves. 



Of these characters, the ones 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 may be ossified. In others, the 



