The Ganoids 3 



perfect definition. In most but not all of the species the tail 

 is distinctly and obviously heterocercal, the lack of symmetry 

 of the tail in some Teleosts being confined to the bones and not 

 evident without dissection. Most of the Ganoids have the 

 skeleton still cartilaginous, and in some it remains in a very 

 primitive condition. Usually the Ganoids have an armature 

 of bony plates, diamond-shaped, with an enamel like that 

 developed on the teeth. In all of them the pectoral fin has 

 numerous basal bones or actinosts. All of them have the air- 

 bladder highly developed, usually cellular and functional as a 

 lung, but connecting with the dorsal side of the gullet, not with 

 the ventral side as in the Dipnoans. In all living forms there 

 is a more or less perfect optic chiasma. These ancient forms 

 retain also the many valves of the arterial bulb and the spiral 

 valve of the intestines found in the more archaic types of fishes. 

 But traces of some or all of these structures are found in some 

 bony fishes, and their presence in the Ganoids by no means 

 justifies the union of the Ganoids with the sharks, Dipnoans, 

 and Crossopterygians to form a great primary class, Pal&ich- 

 thyes, as proposed by Dr. Gunther. Almost every form of body 

 may be found among the Ganoids. In the Mesozoic seas these 

 fishes were scarcely less varied and perhaps scarcely less abundant 

 than the Teleosts in the seas of to-day. They far exceed the 

 Crossopterygians in number and variety of forms. Transitional 

 forms connecting the two groups are thus far not recognized. So 

 far as fossils show, the characteristic actinopterous fin with its 

 reduced and altered basal bones appeared at once without in- 

 tervening gradations. 



The name Ganoidei (ydvos, brightness; eidos, resemblance), 

 alluding to the enameled plates, was first given by Agassiz to 

 those forms, mostly extinct, which were covered with bony scales 

 or hard plates of one sort or another. As the term was originally 

 defined, mailed catfishes, sea-horses, Agonidce, Arthrodires, 

 Ostracophores, and other wholly unrelated types were included 

 with the garpikes and sturgeons as Ganoids. Most of these 

 intruding forms among living fishes were eliminated by Johannes 

 Miiller, who recognized the various archaic characters common 

 to the existing forms after the removal of the mailed Teleosts. 

 Still later Huxley separated the Crossopterygians as a distinct 



