FAMILY CHARACTERISTICS. 119 



the whole shell, or, when 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 (PicidcB) and the 

 Parrots (Psittacidce) , 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 



