248 The Ganoids 
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, Paletch- 
thyes, as proposed by Dr. Gtinther. 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 (yavos, brightness; e7dos, 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, Agonide, 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 
Muller, 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 
