318 AMERICAN TESTUDINATA. Part II. 



what was the real value of all these divisions. Trusting, in a measure, to the 

 principles discussed in the second chapter of the first part of this work, I soon 

 ascertained which of them exhibit generic characters, and which were to be con- 

 sidered as families. I may well add, that I had also the gratification of finding 

 that the natural groups, which I had thus practically circumscribed, afforded new and 

 additional evidence of the correctness of the general principles ascertained before 

 by a more extensive study of other classes. This direct confirmation of the gen- 

 eral views there expressed shows plainly that these principles are likely to be of 

 immediate practical use in the special investigation of any type of the animal 

 kingdom, and may pai'ticularly assist zoologists in finding out the prominent char- 

 acters of any kind of natural groujDS of animals. 



In the following pages, I have attempted to show how, according to these 

 principles, fiimilies ought to be characterized. It will be seen, I hope, that, though 

 it is easy to acquire satisfactory evidence that families are distinguished one from 

 the other by distinct forms, it requires the most careful comparison to discover 

 what are the structural elements which constitute these different patterns. And if 

 this be so, it must be obvious, that such investigations necessarily lead to inter- 

 esting results respecting the meaning of the structural differences which distinguish 

 them. For my own part, I have already satisfied myself that in this way much 

 can be learned of the habits of animals, the mode of life being in dii-ect rela- 

 tion to the form of the animal. More than once already has the direct obser- 

 vation of the habits of our Turtles confirmed what the study of their form had 

 at first only led me to suspect. 



The essential elements of the form of Testudinata, as far as the body is con- 

 cerned, are, first, the curve of the back, following the line of the vertebral column, 

 and its relation to a similar line along the middle of the lower surface ; secondly, 

 the outline of the outer edge of the shield, in its relation to the height of the 

 carapace, and the depth of the lower part of the body; thirdly, the connection 

 of the upper and the lower surface of the body, as determined by the lateral 

 curves of the carapace and the plastron ; fourthly, the outline of the plastron in 

 connection with the openings through which the head, the limbs, and the tail are 

 protruded between the vipper and the lower parts of the shield; fifthly, the rela- 

 tion of the bulk of the body -with reference to the longitudinal axis. Next to 

 these elements, the form of the neck and head affords excellent characters, as 

 weU as the form of the limbs, the relations of the front and hind pair, the 

 articulation of their joints, and especially the form of the feet, the mode of con- 

 nection of the toes, and the manner in which they act upon the medium of 

 resistance when the animal is in motion. 



It has already been stated above, that though orders form necessarily progres- 



