THE CHARACTERS OF GENERA. 133 
Worms, the different Genera of the Leech Fam- 
ily are combined upon the form of the disks by 
which they attach themselves, upon the number 
and arrangement of their eyes, upon the struc- 
ture of the hard parts with which the mouth is 
armed, etc. Among Cephalopods, the Family of 
Squids contains several Genera distinguished by 
the structure of the solid shield within the skin 
of the back, by the form and connection of their 
fins, by the structure of the suckers with which 
their arms are provided, by the form of their 
beak, etc. In every Class, we find throughout 
the Animal Kingdom that there is no sound basis 
for the discrimination of Genera except the de- | 
tails of their structure; but in order to define 
them accurately an extensive comparison of them 
is indispensable, and in characterizing them only 
such features should be enumerated as are truly 
generic ; whereas, in the present superficial meth- 
od of describing genera, features are frequently 
introduced which belong not only to the whole 
Family, but even to the whole Class which in 
cludes them. 
