1 8 VERTEBRATE ANIMALS OF THE UNITED STATES 



The paired fins are the pectoral fins, which are anterior in position, 

 and the ventral fins, which are posterior. The pectoral fins are imme- 

 diately back of the head, either on the side of the body or on the breast. 

 The ventral fins (Fig. 3) lie on the belly in front of the anus in the more 

 primitive fishes, in which case they are said to be abdominal in position; 

 in the more specialized fishes they are situated nearer the head, and are 



Fig. 2. — Types of tails of fishes: a, heterocercal tail of sturgeon (Acipenser); b, hetero- 

 cercal tail of garpike (Lepisosteus) ; c, homocercal tail of pike-perch {Slizosledion) {from 

 Fishes of Illinois). 



said to be thoracic in position when they are immediately beneath the 

 pectoral fins and jugular when they are in front of the pectoral fins. 



The fins are stiffened and supported by slender bony rods called 

 the fin-rays (Fig. 4). These are of two kinds, the soft rays, which are 

 jointed and have divided or split terminal ends, and the spiny rays, 

 which are stiff and not jointed and have undivided and sharp-pointed 

 ends. 



The position of the fins and the number of soft and spiny rays are 

 of great importance in descriptions of fishes. The number of the soft 

 rays is expressed by Arabic and that of the spiny rays by Roman numer- 

 als: thus, the expression ''rays of dorsal fin VI, 12" means that the 

 dorsal fin has 6 spiny and 12 soft rays. Where two dorsal fins are 

 present the number of the rays of the anterior fin is separated from that 

 of the posterior one by a dash. Thus the expression "rays of dorsal 



