Isospondyli 265 
Order Isospondyli—Of the various subordinate groups of 
bony fishes, there can be no question as to which is most primi- 
tive in structure, or as to which stands nearest the orders of 
Ganoids. Earliest of the bony fishes in geological time is the 
order of Isospondyli (tc0s, equal; oxorvdvios, vertebra), contain- 
the allies, recent or fossil, of the herring and the trout. This 
order contains those soft-rayed fishes in which the ventral 
fins are abdominal, a mesocoracoid or precoracoid arch is de- 
veloped, and the anterior vertebre are unmodified and essen- 
tially similar to the others. The orbitosphenoid is present in all 
typical forms. In certain forms of doubtful affinity (Zmzom:) the 
mesocoracoid is wanting or lost in degeneration. Through 
the Isospondyli all the families of fishes yet to be considered 
are apparently descended, their ancestors being Ganoid fishes 
and, still farther back, the Crossopterygians. 
Woodward gives this definition of the Isospondyli: ‘ Noto- 
chord varying in persistence, the vertebral centra usually com- 
plete, but none coalesced; tail homocercal, but hamal supports 
not much expanded or fused. Symplectic bone present, mandible 
simple, each dentary consisting only of two elements (dentary 
and articulo-angular), with rare rudiments of a splenoid on the 
inner side. Pectoral arch suspended from the cranium; pre- 
coracoid (mesocoracoid) arch present; infraclavicular plates 
wanting. Pelvic (ventral) fing abdominal. Scales ganoid only 
in the less specialized families. In the living forms air-bladder 
connected with the cesophagus in the adult; optic nerves decus- 
sating (without chiasma), and intestine either wanting spiral 
valve or with an incomplete representative of it.” 
The Classification of the Bony Fishes.—The classification of 
_ fishes has been greatly complicated by the variety of names 
applied to groups which are substantially but not quite identical 
one with another. The difference in these schemes of classi- 
fication lies in the point of view. In all cases a single character 
must be brought to the front; such characters never stand 
quite alone, and to lay emphasis on another character is to 
make an alteration large or small in the name or in the bounda- 
ries of a class or order. Thus the Ostariophysi with the Iso- 
spondyli, Haplomi, and a few minor groups make up the great 
division of the Abdominales. These are fishes in which the 
