FINS. ' 41 



which it is nearly symmetrical it is frequently prolonged into 

 an upper and lower lohe, its hind margin being concave or more 

 or less deeply excised ; in others the hind margin is rounded, 

 and when the middle rays greatly exceed in length the outer 

 ones the fin assumes a pointed form. 



Many and systematically important differences are ob- 

 served in the dorsal fin, which is either spiny-rayed (spinous) 

 {Acantliopterygian), or soft-rayed {Malacoptei^ygian). In the 

 former, a smaller or greater number of the rays are simple 

 and without transverse joints; they may be flexible, or so 

 much osseous matter is deposited in them that they appear 

 hard and truly spinous (Fig. 3) ; these spines form always the 

 anterior portion of the fin, which is detached from, or continuous 



Fig. 4. — Labrax lupus (Bass), an Acantlioi3terygian with anterior spinous, 

 and posterior soft dorsal fin. 



with, the remaining jointed rays. The spines can be erected 

 or depressed at the will of the fish ; if in the depressed posi- 

 tion the spines cover one another completely, their points 

 lying in the same line, the fish is called Jiomacanth ; but if 

 the spines are asymmetrical, alternately broader on one side 

 than on the other, the fish is called heteracanth. The spinous 

 division, as well as the one consisting of jointed rays, may 

 again be subdivided. In the Malacoioterygian type all the 

 rays remain jointed ; indeed, sometimes the foremost ray, with 

 its preceding short supports, is likewise ossified, and a hard 

 spine, but the articulations can nearly always be distinctly 



