FISHES. 91 



all fishes were divided into four groups, ctenoids, cycloids, placoids, and ganoids, based 

 on the character of the scales. When, however, a single fish was found bearing on 

 its body two of these types of scales, the artificiality of this basis of classification 

 was at once recognized, and to-day only the term Ganoidea remains as a memento 

 of the former ideas. To-day it is not one single character which is used to define 

 the group, but all known facts of structure and development, and even then its 

 limits are far from well defined, especially in the direction of the teleosts. Indeed, 

 but one single character can be adduced to separate every ganoid from every teleost ; 

 the presence of a spiral valve in the intestine in the former, and its absence in the 

 latter, and even this does not hold good in all cases, for, besides being found in the 

 lampreys, which we have passed, and in the sharks, which we have yet to study, it 

 exists in but a rudimentary condition in Amia and Lepidosteus. All the other char- 

 acters mentioned below have an exception somewhere. 



The external surf;uo is but rarely Jiaked, but is usually covered with bony scales or 

 plates, which in the sturgeons are large and separate, but in the shovel-nosed sturgeons 

 they form a solid coat of mail envelojDing the tail. Frequently the scales are rhomboidal 

 in outline, but in some they are round and much like those of the teleosts. In some 

 the skeleton remains almost entirely cartilaginous, but in others ossification sets in to 

 a greater or less extent. Thus in some the cartilaginous skull is protected by mem- 

 brane bones, and the framework of the jaws is hardened. In others the ossification 

 extends to the vertebras and to other jjarts. The jjectoral fins usually are well devel- 

 oped, and in the fossil forms attained a peculiar development. The caudal fin is usually 

 heterocercal, that is, the vertebral column extends into the uj)per lobe, the lower lobe 

 remaining much smaller, but in others the two lobes are equal. This point was for- 

 merly much emphasized in the arrangement of fishes, as in the young teleosts a hetero- 

 cercal condition was noticed which disappears in the adult. Mr. Ryder has recently 

 explained, on mechanical grounds, the causes of the heterocercal condition. Frequently 

 one or more of the fins are armed in front with a series of overlapping sjiiny plates, 

 known as fulcra. The existence of these in a fossil is of great value in determining 

 its relationshijjs, and one author has said that " every fish with fulcra on the anteiior 

 margin of one or more fins is a ganoid." 



The heart is furnished with a bulbus arteriosus, which pulsates and is internally 

 furnished with a number of valves, the purpose of which is to prevent the blood flow- 

 ing back during the pause in the beat. An air-bladder is present, and is furnished 

 with a duct leading to the oesophagus. The optic nerves do not simply cross, as in 

 the teleosts, but unite to form a chiasma like that occurring in the higher verte- 

 brates. 



The ganoids are an ancient grou]), well develojied in the paleozoic rocks, but now 

 dying out. The fossil genera are numerous, and the species frequently highly differ- 

 entiated, but to-day only eight genera and between thirty and forty species comprise 

 the ganoid fauna of the world. With its great antiquity and generalized condition, 

 the group presents resemblances to many otliers. On the one hand some of its mem- 

 bers approach the Holocephali among the Elasmobranchs, while, on the other, forms 

 like Amia show a distinct teleostcan tendency. Still others appear to reach out 

 towards the lung fishes. 



Seven orders of ganoids are recognized, three of which, Acanthodini, Placodcrmi, 

 and Pycnodontina, are extinct. They appeared ip the Silurian and Devonian rocks, 

 and repi-esented the ichthyc type in the age of fishes. 



