4 PKBLIMINABY EEPOET 



borders the cheek on its posterior and ventral edges and whicli in 

 the perch is toothed or serrate. Under the preopercle and in front 

 of the subopercle is the interopercle (Fig. I., I. Op.) The bone just 

 in front of the eye is the preorMtal (Fig. I., P. Orb.); the one just be- 

 low the eye is the suborhital, and this bone sometimes bears a iony 

 stay which extends across the cheek towards the preopercle. 



The mouth varies greatly in the different species of fish, but 

 that of the perch may be considered typical. Each side of the 

 upper jaw is formed by the premaxillary bone which bears teeth. 

 (Fig, I., P. M.). At the middle of the snout there is a backward, 

 projection on the premaxillary, and when this and the remainder 

 of the bone is fastened by the skin and muscles alone the pre^ 

 maxillaries can be drawn forward, and are then said to be pro- 

 tractile. This character is often essential in identification. Back 

 of the premaxillary, on each side of the head, is the maxillary (Fig. 

 I., M.). This bone varies greatly in the different families. Thus 

 in the cat-flsh family it is much absorbed and ends in a long fleshy 

 filament called a barlel, while in the perch it is well developed. In 

 the bass and sunfish there is an additional supplemental bone on 

 the upper side of the maxillary (Fig. II., S. M.), the presence or ab- 

 sence of which is essential in determining the species of sun-fish. 

 The dentary (tooth bearing) bones form the lower jaw. The mouth 

 is said to be inferior when drawn down like that of the sucker, 

 horizontal when about on a line with the axis of the body, and oblique 

 ,when opening at an angle like that of the perch and bass. 



The thin membrane under the opercle and subopercle is the 

 branchiostegal membrane, and the curved bones in it are the branchi- 

 ostegal rays, which, in the perch, number seven. The number of 

 these rays is often an essential character. The narrow part of the 

 body near where the branchiostegal membranes of each side join 

 is called the isthmus and when the gill membranes are joind to it 

 (Fig. III.) thy are said to be unAted to the isthmus, as is the case in 

 the red horse sucker. When joined to each other and not directly 

 to the body, as in the perch, they are said to be free from the 

 isthmtis. 



For the reason that the size of the mature fish is an indefinite 

 thing most of the measurements used are comparative. The length, 

 however, is given and, when so, it is supposed to be the average 

 length of the mature fish. This length is the distance from the tip 

 of the snout to the base of the caudal fin, or end of the spinal column, 

 and not to the tip of the tail. Head in length means the number of 

 times the distance from the tip of the snout to the posterior edge 



