22 



VERTEBRATE ANIMALS OF THE UNITED STATES 



lateral portions of the premaxillary, on each side. The premaxillaries 

 are, in many fishes, protractile that is, they can be thrust forward. 

 In certain fishes a supplementary maxillary lies above the posterior 

 portion of the maxillary, on each side. 



_ ^ . ^ ^ . ^ ^ 10 II 12 



Fig. 7. — Head of a fish: i, nostrils; 2, premaxillary; 3, mandible; 4, lateral portion of 

 premaxillary; 5, maxillary; 6, supplementary maxillary; 7, cheek; 8, interopercle; 9, pre- 

 opercle; 10, branchiostegalis; 11, subopercle; 12, opercle. 



Many species of fishes, catfish for instance, have one or more 

 pairs of elongated tentacles called barbels extending from one or both 

 jaws (Fig. i). 



® 



Fig. 8.- 



-Position of the mouth: a, mouth terminal in position; b, mouth superior; c, mouth 

 inferior (from Siissw. Fauna Deut.). 



Below and behind the eyes on each side of the head is the cheek. 

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

 them Kes the interopercle and the subopercle: these structures form the 



