ICHTHYOPSIDA. 231 
pectoral and ventral fins represent, as just said, the fore and 
hind limbs, and consequently there are always fwo of each, 
when they are present at all. They are, therefore, spoken of as 
the “paired” fins. Besides these, however, or in the absence 
of one or other of these, there is also a series of what are called 
‘““median” fins; that is to say, fins which are placed in the 
middle line of the body, and which are unpaired, having no 
fellows. ‘These median fins agree with the paired fins in being 
expansions of the integument, supported by bony or gristly 
supports or “rays,” and they are carried by the heads of the 
“interspinous ” bones, already described (fig. 123,72). Theyare 
variable in number, and in some cases there is only a single 
fringe running round the hinder extremity of the body. Com- 
monly, however, the median fins consist of one or two expan- 
sions of the dorsal integument, called the “ dorsal” fins (fig. 123, 
d ad’); one or two on the ventral or lower surface near the vent, 
called the “‘ anal” fins (2); and a broad fin at the extremity of 
the vertebral column, constituting the “caudal” fin or tail (¢). 
The tail in all fishes is placed vertically —that is to say, 
it strikes the water laterally or - 
from side to side, and it is the 
chief organ of progression in the 
fish, Two very distinct types of 
tail are found amongst the fishes. 
In one of these, found in most 
living forms, the tail is composed 
of two nearly equal lobes which 
spring from the end of the spine 
(fig. 125, a). This form of tail is 
said to be “homocercal.” In 
the other type of tail, found in 
the dog-fishes, sharks, and other 
living fishes, as well as in many 
extinct forms, the tail is un- 
equally lobed, and is said to be Te jes Tal of diirnt Fibes 
heterocercal” (fig. 125, 0). In Heterocercal tail (Sturgeon). 
these forms the vertebral column 
is prolonged into the upper lobe of the tail, and the greater 
portion of the tail is found below the spine. 
In both the paired and the median fins the integument is 
supported by a series of spine-like bones, which are called 
