40 Isospondyli 



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, Hemibrancliii), 

 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 Teleocephali, 

 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 Miiller 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 vertebras 

 unaltered (as distinguished from the Ostariophysi], 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 Nematognathi, Plectospondyli, or Apodes. 

 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 Iniomi, which lack the mesocoracoid and the 



