FAMILY CHARACTERISTICS. 119 
the whole shell, or, whén equal to the lower side 
and connected with a great height of the body, 
gives it a quadrangular form, or, if the height is 
reduced, produces an elongated form, or, finally, 
a rounded form, if the passage from one side to 
the other is gradual. A comparison of the po- 
sition of the internal organs of different Species 
of Unios with the outlines of their shells will 
leave no doubt that their form is determined by 
the structure of the animal. 
A few other and more familiar examples may 
complete these remarks. Among Climbing Birds, 
for instance, which are held together as a more 
comprehensive group by the-structure of their 
feet and by other anatomical features, there are 
two Families so widely different in their form 
that they may well serve as examples of this 
principle. The Woodpeckers (Picide) and the 
Parrots (Psittacide), once considered as two 
Genera only, have both been subdivided, in con- 
sequence of a more intimate knowledge of their 
generic characters, into a large number of Gen- 
era; but all the Genera of Woodpeckers and all 
the Genera of the Parrots are still held together 
by their form as Families, corresponding as such 
to the two old Genera of ‘Picus and Psittacus. 
They are now known as the Families of Wood- 
peckers and Parrots; and though each group in- 
cludes a number of Genera combined upon a 
