i8 



VERTEBRATE ANIMALS OF THE UNITED STATES 



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 hn \T, 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 



fins XI-III, 5 " means that the anter- 

 ior dorsal fin has 11 spiny rays and 

 the posterior dorsal has 3 spiny and 5 

 soft rays. In certain groups of soft- 

 rayed fishes, such as the trout, catfish 

 and others, the posterior dorsal fin is 

 a fleshy structure without rays of any 

 kind, and is consequently not men- 

 tioned in such descriptions; it is called 

 the adipose fin. 



Lateral Line (Fig. i). — In most 

 fishes the lateral line appears as a 

 longitudinal series of modified scales 



Fig. 3. — Position of the ventral fins: 

 {top figure), the ventral fins in an ab- 

 dominal position; {middle figure), in a 

 thoracic position; {bottom figure), in 

 a jugular position: i, pectoral fins; 2, 

 ventral fins {from Sussw, Fauna. Dent.). 



Fig. 4. 



—Soft fin-rays (o) ; spiny fin-rays {b) 

 {from, Siissw, Fauna. Deut.). 



extending along the side of the body between the head and the caudal 

 fin. In the lower, soft-rayed fishes it tends to run parallel to the line 

 of the belly, and in the higher, spiny-rayed fishes to the line of the 

 back. In many fishes it is not complete or is altogether wanting, at 

 least so far as any external appearance is concerned. 



Scales (Fig. 5). — In some species of fishes, the catfish for example, 

 the skin is naked and without any hard skeletal covering. In most 

 fishes, however, the skin is protected by a layer of scales, and in a few 

 groups by bony plates, prickles, or other skeletal structures. By 



