8 NEW SOUTH WALES 
advance of this, with a free margin like a second gill ; this is called the 
pre-operculum. It varies considerably in shape, often having a toothed 
edge or spines, and so becomes useful as a feature in classification. The 
term operculum is, as already said, only applied to the hind margin of the 
gill-cover, which is divided into movable segments. The under one is 
called the swb-operculum, and the segment. above is called the inter-oper- 
culwm ; they are separate bones. These bones are sometimes rudimen- 
tary and sometimes absent. All of them are frequently referred to 
in descriptions of fishes in scientific works. 
Everyone must have noticed along the sides of most bony fishes a line 
something like a division between the belly and back. It is sometimes 
straight, but more often is curved in the most varied manner. This is 
called the lateral line, and is caused by a series of perforated scales 
through which mucus is especially secreted, though no doubt all the sur- 
face of a fish secretes the same fluid to some extent. This perforated 
line is provided with abundant nerves, and is called the MuUciferous 
system. Some fishes have many lateral lines, and our coast and river 
mullets have none at all. 
All fishes are of distinct sex. The females in the majority of instances 
are oviparous. A few bear the young alive; generally the eggs are 
deposited by the female and are afterwards fertilized by the male. In 
the males, lying along the intestines, there are two soft bodies called the 
milt. In certain seasons these contain a milky secretion, which is the 
fertilizing fluid. This is deposited on the eggs by the male, or merely 
injected into the water. A very small quantity of this fluid is sufficient 
for the impregnation of an immense number of ova, and it is owing to 
this circumstance that the artificial impregnation and hatching of fishes 
is easily practised and immense numbers of fish preserved. 
In the same relative position as the milt (soft roe) in the male are 
found the bodies called the hard roe in the female. This is a mass of 
unimpregnated eggs. 
In their young states fishes differ so much from the forms they assume 
in full growth that they have been very frequently mistaken for distinct 
species. Though a number of such mistakes have been rectified of late 
years, there are doubtless still some received species which are only the 
young of other forms. Until all the stages of growth are known these 
errors will not be detected. 
Having now dealt with all the most necessary elements of scientific 
classification, we can apply them to the general divisions which are 
made. Let it be premised, however, that those first entering upon this . 
study must not be disheartened if they find it difficult to identify with 
certainty some of the details. Though perhaps a little out of place in 
such a work as this, it may be well to suggest a method by which the 
details of scientific description may be mastered. Instead of taking an 
unknown fish and trying to make it out by the aid of a catalogue, such 
as Giinther’s or Macleay’s, let the student compare a fish, the scientific 
name of which he is sure of, with the description given of it in these 
works. When this has been done in the case of ten or a dozen species of 
different families, the work of identification will be comparatively easy. 
All who are in reach of the Museum in Sydney, or any of our colonial 
