6 NEW SOUTH WALES 
complete. In nearly all the Palwichthyes there is a peculiar arrange- 
ment for the termination of the tail, which is called heterocercal. The 
vertebree are continued into the upper lobe of the tail, which makes it 
unsymmetrical. Inv Teleostean the caudal fin is more or less symmetrical 
on each side of the end of the vertebre. This is called homocercal. In 
this the caudal fin appears to be centred on the last vertebra, which 
coalesces with a flat bony plate, the hypural bone, on the flat margin of 
which the fin rays are placed. 
The cartilaginous skeleton of the Paleichthyes is peculiar in leaving 
no sutures in the skull, which is always formed of a single piece, but in 
which we can distinguish, by means of the projections, grooves, hollows 
and holes, certain regions which are analogous to the bones in the skulls 
of other fishes. Again, parts of the vertebre in certain rays are united 
into a single body. The gelatinous substance which in other fishes fills 
the intervals of the vertebre and communicates from one to the other 
only by a small hole, forms in several Chondropterygti a cord (notochord) 
which runs through all the bodies of the vertebra, almost without 
varying in diameter. 
As already stated, most of these details will be useless to the 
ordinary observer for the purposes of classification. | Other portions of 
the body of a fish must be relied upon. The chief of these are the fins. 
*They are supposed to correspond to the limbs of other vertebrate 
animals, Those corresponding with the fore limbs are said to be 
pectoral, and those which correspond to the hind limbs are called 
ventral. Vertical fins on the back are said to be dorsal, and 
similar fins underneath, in a line with the axis of the body and 
near the vent, are called anal. The tail fin is called the caudal. 
Any of these may be present or absent; sometimes they are highly 
developed and extend a great length, or again may be a mere fold 
in the skin; thus the dorsal fin may extend from the head to the 
vent, joining the anal and caudal in one continuous line. Many 
important differences are perceived in the dorsal fin on which systems 
of classification are founded. The rays which support it may contain 
spines of bone, or they may be all soft and jointed. The fishes with 
spinous rayed dorsal fins comprise an immense order called the 
Acanthopterygit or spiny fins. The soft-rayed are called Malacop- 
terygians. These spines can nearly always be erected or depressed at 
the will of the fish. If, when depressed, they cover each other com- 
pletely so that their points all lie in the same line they are called 
homocanth ; but if they are unsymmetrical, that is alternately broader 
on one side than on the other, they are called heterocanth. The anal 
fin may be divided into one or more fins, or be absent. In Acanthop- 
terygians its foremost rays are frequently spinous. 
It must be borne in mind also that though the fore and hind limbs 
are represented by fins in fishes, yet in most cases they are close together 
and generally near the head. The pectorals or fore limbs with their 
bony supports are always fixed immediately behind the gill-opening, 
The ventrals are subject to much variation in position, and formerly 
were used by zoologists to distinguish large families of fishes by their 
position, ‘When inserted behind the pectorals on the abdominal surface, 
the fishes were grouped into one order under the old Linnean arrange- 
ment, called Abdominales, This included salmon, silurus, pike, 
