Ix FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. V. 



ities from which Mexican fishes have been taken are given with each 

 reference. The names of places mentioned in parentheses preceding 

 each description are localities from which the species was taken by 

 me in 1903. In many cases the localities of the early authors are very 

 indefinite. Frequently "Mexico" only is mentioned. A few of the 

 definite localities as given which appear to be wrong are discussed 

 under the species in question, it being intended that some one may 

 be able to either verify these or to correct them. 



DEFINITION OF THE TWO CLASSES OF FISHES OR FISH- 

 LIKE VERTEBRATES INHABITING THE FRESH 

 WATERS OF MEXICO. 



The fresh- water fishes, or fish-like vertebrates of Mexico, belong 

 to two classes: the Marsipobranchii, Lampreys, hag-fishes, and the 

 like; and Pisces, the sharks, skates, and the true fishes. The former 

 is represented by a single species of lamprey known at present only 

 from the Lerma River Basin; the latter is represented by a large 

 number of species of true fishes. Sharks and skates often ascend 

 streams far above tide-water, but at present we have no evidence 

 that any of these forms are permanent residents of the rivers of 

 Mexico. 



The fishes, or fish-like vertebrates, may be defined as cold- 

 blooded vertebrates adapted for life in the water, breathing by means 

 of gills, which are persistent throughout life, and having the limbs, 

 if present, developed as fins, never with fingers and toes. The cere- 

 bral hemispheres are smaller than the optic lobes. 



The two classes of fish-like vertebrates may be briefly defined as 

 follows : 

 CLASS I. MARSIPOBRANCHII (THE LAMPREYS). 



Skull imperfectly developed, without true jaws; gills purse- 

 shaped, not attached to cartilaginous arches; a single 

 median nostril; body eel-shaped. 

 CLASS II. PISCES (THE FISHES). 



Skull well developed, and with jaws; gills attached to 

 arches; nostrils not median, in one or more pairs. 



ARTIFICIAL KEY TO THE FAMILIES OF , MEXICAN 

 FRESH-WATER FISHES. 



a. Mouth subcircular, without true jaws; 7 gill PAGE 



openings on each side; no paired fins Petromyzontida i 



aa. Mouth normal, with true jaws; one gill open- 

 ing on each side. 



