Isospondyli 267 
are not strongly differentiated, their distinctive characters are 
real, ancient, and important, while those of the aberrant groups, 
called orders by Gill (as Plectognathi, Pediculati, Hemibranchit), 
are relatively modern and superficial, which is one reason why 
they are more easily defined. There seems to us no special 
advantage in the retention of a central order Teleocephalt, 
from which the divergent branches are separated as distinct 
orders. 
While our knowledge of the osteology and embryology of 
most of the families of fishes is very incomplete, it is evident 
that the relationships of the groups cannot be shown in any 
linear series or by any conceivable arrangement of orders and 
suborders. The living teleost fishes have sprung from many 
lines of descent, their relationships are extremely diverse, and 
their differences are of every possible degree of value. The 
ordinary schemes have magnified the value of a few common 
characters, at the same time neglecting other differences of 
equal value. No system of arrangement which throws these 
fishes into large groups can ever be definite or permanent. 
Relationships of Isospondyli—For our purposes we may divide 
the physostomous fishes as understood by Muller into several 
orders, the most primitive, the most generalized, and economically 
the most important being the order of Isospondyli. This order 
contains those bony fishes which have the anterior vertebrze 
unaltered (as distinguished from the Ostariophyst), the skull 
relatively complete, or at least not eel-like, the mesocoracoid 
typically developed, but atrophied in deep-sea forms and finally 
lost, the orbitosphenoid present. In all the species the ventral 
fins are abdominal and normally composed of more than six 
rays; the air-duct is developed. The scales are chiefly cycloid 
and the fins are without true spines. In many ways the order 
is more primitive than Nematognatht, Plectospondyli, or A podes. 
It is certain that it began earlier in geological time than any 
of these. On the other hand, the Isospondyli are closely con- 
nected through the Berycoidei with the highly specialized fishes. 
The continuity of the natural series is therefore interrupted by 
the interposition of the side branches of Ostariophysans and eels 
before considering the Haplomi and the other transitional forms. 
The forms called Iniom?, which lack the mesocoracoid and the 
