lviii Field Columbian Museum — Zoology, Vol. V. 



4" or "Head 4^3 " indicates that the head of the fish is contained 4 

 times or 4% times in the distance from the tip of the snout to the end 

 of the last caudal vertebra; "Depth 4" that the greatest depth (none 

 of the fins being included) is contained 4 times in the same distance ; 

 " D. 8," indicates that the fish has a single dorsal fin which is composed 

 of 8 soft rays; "D. iv, 9," that the dorsal fin is single and is composed 

 of 4 spines and 9 soft rays; "D. iv-9," that there are two dorsal fins, 

 the first one composed of 4 spines and the other of 9 soft rays. Spines 

 are always indicated in roman letters, soft rays by figures. The ab- 

 breviations used in the count of other fin rays and spines are similarly 

 explained. The diameter of the eye, the length of the snout, and 

 many other short measurements are compared with the length of the 

 head. "Eye 3 in head," "Snout 3 in head," indicate that each 

 is contained 3 times in the length of the head. In these particular 

 cases " yi of the length of the head ' ' would mean the same thing. 



All of these measurements are so far as possible intended to apply 

 to mature fish of average size; a certain amount of allowance must, 

 however, be made for age and individual variation. Young fishes 

 usually have larger eyes, shorter snout, smaller mouth, and longer fin 

 rays than adults of the same species; more often they are also deeper, 

 but this is not always true. 



At the close of each description the approximate length of the 

 adult of the species is given. The size of fishes is more dependent 

 on environment than in case of any other group of vertebrates. 

 Fishes in large bodies of water grow much more rapidly and larger 

 than the same species under other conditions. If food is plentiful 

 and easily obtained the fish will eat oftener than where less favored. 

 It is not necessary for a fish to eat as much as one meal each day ; he 

 may eat once each week, or even once each month, without appar- 

 ently experiencing the evil effects of hunger, but in such cases his 

 growth will be somewhat retarded. 



For convenience in classification, and to afford an expression of 

 relationship, fishes, or fish-like vertebrates, are divided into classes, 

 each class into orders, orders into genera, and each genus com- 

 prises one or more species. 



The catfish of the Rio Balsas is classified as follows : 



Class, Pisces. 



Order, Nematognathi. 



Family, Silurid^e. 



Genus, Istlarius. 



Species, balsanus. 

 Each of these divisions may be divided into sub-groups. 



