Introduction 



and with a lower jaw; with the limbs present and developed as 

 fins, or rarely wanting through atrophy; having the exoskeleton 

 developed as scales or bony plates or horny appendages; and with 

 the median line of the body with one or more fins composed of 

 cartilaginous rays connected by membranes. 



But a still narrower definition is sometimes necessary, and 

 we may separate from the true fishes the various lower types 

 developed before the formation of the paired fms and jaws of the 

 fishes of to-day. 



The sharks are not true fishes, for they have no membrane- 

 bones or gill-covers, and the upper jaw is simply the front of the 

 palate, no upper jaw-bones being developed. 



The same is true of the skates, the chimaera and the lung- 

 fishes. The lung-fish, like the bichir of the Nile, another fish- 

 like creature, not a true fish, has, instead of pectoral fins, long- 

 jointed appendages with a fringe of rays along the side. From 

 the structure, as seen in the bichir (Polypterus bichir), it is not 

 a great change to the forked limbs of the frog, and it is from air- 

 breathing amphibious fishes like these that the original salaman- 

 ders and frogs of the coal measures were descended. All these 

 forms, as well as the mailed and helmeted monsters of the 

 Devonian, are fishes in the broad sense of the term, but not in 

 the narrow one of "true fishes." A true fish is an aquatic ver- 

 tebrate fitted for life in the water, breathing by means of gills, 

 having brain, skull, and lower jaw, the upper jaw formed origin- 

 ally of at least two pieces (premaxillary and maxillary), one on 

 each side, with developed limbs, the pectoral and ventral fins being 

 composed of fin-rays not attached to an elongate jointed axis. 



All of those mentioned in this book are true fishes, and each 

 one can verify this definition, although in a few of them the ex- 

 ternal parts or fin-rays of pectoral or ventral limb are lost altogether. 



The nomenclature and arrangement of species in this work 

 agree essentially with that adopted by the present writers 

 in their "Fishes of North and Middle America," with such changes 

 and modifications as more recent investigations and studies seem 

 to require. Perhaps the most important departure from that work 

 is in the use of fewer trinomial names. This is especially to be 

 noted among the Salmonidce. Usually the Sebago salmon and the 

 ouananiche have been regarded as subspecies of the Atlantic 

 salmon and have been given trinomial names Salmo salar sebago 



XXVll 



