6 NEW SOUTH WALES 



complete. In nearly all the Pakeichthi/es there is a peculiar arrange- 

 ment for the termination of the tail, which is called heterocercal. The 

 vertebrse are continued into the upper lobe of the tail, which makes it 

 unsymmetrical. In Teleostean the caudal fin is more or less symmeti'ical 

 on each side of the end of the vertebrte. This is called homoc&rcal. 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 Palceiclithyes 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 vertebrae in certain rays are united 

 into a single body. The gelatinous substance which in other fishes fills 

 the intervals of the vertebrae and communicates from one to the other 

 only by a small hole, forms in several Chondropterygii a cord (notochord) 

 which runs through all the bodies of the vertebrpe, 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 

 AcanthojJteo-ygii or spiny fins. The soft-rayed are called Malacop- 

 terygians. These spines can neai^ly 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, 



