38 Isospondyli 



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 (tiros, equal; o-TrovdvXos, 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 vertebrae are unmodified and essen- 

 tially similar to the others. The orbitosphenoid is present in all 

 typical forms. In certain forms of doubtful affinity (Iniomi) 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 haemal 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) fins abdominal. Scales ganoid only 

 in the less specialized families. In the living forms air-bladder 

 connected with the oesophagus 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 



