FTSHES. 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 (ianoidea I'emains 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 ])resence of a sj)iral 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 Z,epidoiiteics. All the other char- 

 acters mentioned below have an exception somewhere. 



The external surface is but rarely naked, but is usually covered with bony scales or 

 plates, wliicli in the sturgeons are large and separate, but in the shovel-nosed stui-geons 

 they form a solid coat of mail enveloping 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 vertebrte and to other parts. The pectoral fins usually are well devel- 

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

 heterocercal, that is, the vertebral column extends into the upper 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 sei'ies of overlapping spiny plates, 

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

 its relationships, and one author has said that " every fish with fulcra on the anterior 

 margin of one or more fins is a ganoid." 



The heart is furnished with a bulbus arteriosus, which ])ulsates and is internally 

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

 ing l)ack during the pause in the beat. An air-bladder is jiresent, and is furnished 

 with a duct leading to the oesojihagus. The ojttic 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 group, well developed in the ])aleozoic 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 com]>rise 

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

 the group presents resemblances to many others. On the one liaiid some of its mem- 

 bers approach the Ilolocephali among the Elnsmobranchs, while, on the other, forms 

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

 towards the lung fishes. 



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

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

 and represented the ichthyc type in the age of fishes. 



