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. 



