LABIPREYS AND FISHES OF INDIANA. 155 



will reveal the structural differences The scales of fishes are arranged 

 in more or less regular rows, transverse and longitudinal. The number 

 of these furnish specific characters. In most fishes one of the rows which 

 runs along the side is made up of Fcales which have each a small pore. This 

 row of pores is called the lateral line. The number of transverse rows of 

 scales may be determined by counting the number of scales along this 

 lateral line, beginning just behind the gill-opening. The number of longi- 

 tudinal rows may be determined by counting from the anterior of the 

 dorsal fin to the lateral line and from the lateral line to the middle of the 

 belly. The number of rows, both transverse and longitudinal, is some- 

 what variable, even in the same species. The formula, " scales, 6 — 45 — 

 12," means that there are forty-five rows of scales along the lateral line, 

 six above it and twelve below it. 



The fins of fishes are folds of membrane supported by firm cartilagin- 

 ous or bony rays. They are classed as vertical or unpaired and horizontal 

 or paired. The vertical fins stand in a vertical plane along the middle 

 line of the trunk and tail. The rays are of two kinds, soft, or articu- 

 lated, and spiny. The soft rays are distinguished by being cross-jointed, 

 and often by being split toward their extremities. The spines are neither 

 jointed nor do they divide. They may be strong and inflexible or they 

 may be slender and weak. Close examination with a lens may some- 

 times be necessary in order to distinguish the weak spines from undivided 

 or mutilated soft rays. The two kinds may be studied in the vertical 

 fins of a bass or of a sun-fish. When both soft and spiny rays are pres- 

 ent, the latter occupy the anterior portion of the fiu. The fin may con- 

 sist wholly of soft rays or wholly of spines. 



The vertical fins are known as the dorsal (along the back), the anal 

 (along the under side of the tail), ard caudal (at the tip of the tail). In 

 some cases, as in the eel, the dorsal and anal are confluent with the 

 caudal. The dorsal may be undivided, or it may consist of two or three 

 distinct portions. Occasionally the spinous portion of the fin may be 

 represented by a few spines without membrane to connect them. The 

 number of rays, soft and spinous, in any species is likely to be quite 

 definite, especially when not great. In some fishes, as the cat-fishes, 

 there is a fleshy, rayless fin located behind the proper dorsal. It is 

 called the adipose fin. 



The caudal fin varies greatly in form. Two modifications of it must 

 be noticed. In a few, as the paddle-fish, the sturgeons, gars, and to 

 somewhat less extent in Amia, the spinal column appears to run out 

 along the upper border of the caudal. In adult gars and in Ainia this 

 is shown by the extension of- the scales further back on the upper side of 

 the tail than on the lower. The term heterocercal has been employed to 

 express this condition. In most fishes the backbone appears to stop 



