ON CLASSIFICATION. 
11 
The next division is that of the Genera. 
A genus may he defined, roughly, as a group of 
fishes, which closely resemble one another in all im- 
portant details of then' structure, and differ from all 
other genera in one or more essential points ; it may 
contain only one species, or it may contain a very 
great number ; the genus Barbus, for example, in 
Gunther’s Catalogue, numbers over 160 species. 
It is, as may be supposed, a difficult matter to 
decide what points are of sufficient importance to con- 
stitute a genus, and as almost every writer on the 
subject considers himself qualified to form his own 
opinion, and to bestow names of his own choosing, 
the result is a most embarrassing confusion. 
It must be borne in mind that the members of each 
separate family have a tendency to vary in certain 
respects, and to remain constant in certain other 
points. 
The Siluridee, for instance, are eminently variable 
as to their teeth, the Cyprinidse, on the other hand, 
vary very slightly in this respect. 
ITence it is easy to understand that a slight differ- 
ence in the number of their teeth between two 
Cyprinoid fishes would suffice to indicate that they 
belong to different genera, whereas a much 
greater difference might exist between the teeth of 
two Siluroid fishes who belong to one and the same 
genus ; in their case it might only show that they 
were distinct species. 
It is useful to remember that the first scientific 
