20 



VERTEBRATE ANIMALS OF THE UNITED STATES 



usually an important part of a description. It must be remembered, 

 however, that there is often a considerable variation in the coloration 



of a species in dififerent individuals 

 and at different times of the year, that 

 the two sexes are often colored differ- 

 ently, the female being duller than the 

 male, and that bright colors, especially 

 reds and blues, often fade after the 

 death of the animal or when it has 

 lain in a preserving fluid. 



Head (Fig. 7). — The portion of 

 the head in front of the eyes is the 

 snout. The mouth may be either ter- 

 minal in position, inferior, subinferior 

 or superior, and may also be either 

 horizontal or oblique (Fig. 8). The 

 nostrils are four in number, two on 

 each side immediately in front of the 

 eye. The upper jaw contains the 

 paired premaxillary bones which sup- 

 port its anterior portion, and also often 

 the lateral portions as well, and the 

 paired maxillaries, which lie above and 

 behind the lateral portions of the 

 premaxillary, on each side. The pre- 

 maxillaries are, in many fishes, pro- 

 tractile, that is, they can be thrust 

 forward. In certain fishes a sup- 

 plementary maxillary lies above the 

 posterior portion of the maxillary, on 

 each side. 



Many species of fishes, catfish for 

 _ , , . ^ , , , ., instance, have one or more pairs of 



Fig. 6. — A portion of the lateral side ' ^ 



of a fish's body between the dorsal fin elongated tcntaclcS Called borbels 

 (above) and the anal fin (below) to , j- r u iU • 



illustrate the count of the scales, the exteudmg from oue or both jaws 



lateral line series of which is in the (Fig. i). 



middle, and the dorso-ventral series i i i • j i 



above and below it (from Siissw. Fauna Bclow and behmd the eyes on 

 ^^"'•'*' each side of the head is the cheek. 



Behind the cheek is the preopercle, and behind it the opercle; beneath 

 them lies the interopercle and the suhopercle; these structures form the 

 gill-covers. Ventral to them Hes the hranchiostegal membrane supported 



