Introduction. lix 



The scientific name of a fish is a combination of the name of the 

 genus and the species. The large catfish (Bagre) found in the Rio. 

 Balsas belongs to the genus Istlarius and its specific name is balsanus. 

 Its scientific name is Istlarius balsanus. The scientific names as they 

 are usually written, and as they appear in this work, are followed by 

 the name of the person or persons who first described the fish and 

 proposed the specific name for it. The name referred to above is 

 properly written Istlarius balsanus Jordan & Snyder. 



Under the description of each order, family, and genus a key is 

 given to facilitate the identification of the species. These keys are 

 arranged as far as possible on the alternative basis. To use the key, 

 examine the fish in hand and read the first statement lettered "a"; 

 either that or its alternative, lettered "aa," is true. Next read the 

 lettered statement below "a" or "aa" as the case may be, and con- 

 tinue until a letter is reached under which there are no subdivisions, 

 when you will be brought to an order, family, generic, or specific name. 

 The page indicated by the figure following this name contains the 

 description of the family, genus, or species, as the case may be, to 

 which your fish belongs. If a family, read the key to the genera, then 

 to the species, which will refer you to the description of the species 

 of the fish in question. 



In order to find at once the family to which the specimen in hand 

 belongs it is better to use the artificial key to the Families of Fishes 

 on the following page. 



In all of the keys in this work, except this one, the sequence of 

 the orders, families, genera and species, as the case may be, is the same 

 as the descriptions. It must be borne in mind that this work treats 

 only of fishes known to inhabit the fresh waters of Mexico north of 

 the Isthmus of Tehuantepec. 



Reference is made under each genus to the original description, 

 and the type of the genus. No other reference is given unless the 

 synonym is based on forms found in Mexico or is deserving of 

 subgeneric rank. Under the name of each species the first reference 

 is to the original description and the type locality ; references are also 

 made to Dr. Giinther's Catalogue of Fishes of the British Museum (i), 

 to his Fishes of Central America (2), to Prof. Garman's Cyprinodonts 

 (3), and to Jordan & Evermann's Synopsis of Fishes of North and 

 Middle America (4). All other references relate only to Mexican 

 fresh-water fishes. For aid in geographical study all known local- 



(1) Gunther, Catalogue, Fishes British Museum, 8 vols. 1S59 to 1870. 



(2) Gunther, Fishes of Central America, 1 vol. 1869. 



(3) Garman, The Cyprinodonts, Mem. Mus. Comp. Zool., vol. xix, 1895. 



(4) Jordan & Evermann, Bull. 47, U. S. Nat. Mus. 4 vols. 1896 to 1900. 



