Chap. II. FAMILIES OF TESTUDINATA. 319 



sive series as they are characterized by the degrees of complication of tlieir 

 structure, other kinds of groups may stand higher or lower, when compared with 

 one another.^ This is strikingly the case with the liunilies of Testudinata, between 

 which there is a marked gradation. Their respective standing is even so easily 

 ascertained, tliat, ever since these animals have been divided into families, all her- 

 petologists have arranged them in the same progressive series, beginning with the 

 marine Turtles as the lowest, and ending with the land Turtles as the highest, 

 while they all assign to the fresh-water Turtles an intermediate position between 

 the two other groups. It is true, as far as the marine Turtles, on one hand, 

 and the land and fresh-water families, on the other hand, are concerned, the 

 relative position of these tAvo groups is determined by structural features, which 

 constitute sub-orders ; but the gradation of the families is not limited to the 

 relative standing thus assigned to them, for even the fiimilies of the Chelonii, 

 and those of the Amydte, stand higher or lower among themselves; and within 

 these narrower limits the gradation is no longer determined by the comphcation 

 of their structure, but chiefly by peculiarities in those features Avhich essentially 

 characterize the families, namely, the forms. Chelonii, compared with AmydiB, have 

 lower fonns ; the form of the Sphargidido? is made up of elements of an inferior 

 order to that of the Chelonioida; ; the form of the Trionychida; is simpler in 

 its essential elements than that of the Chelyoida"", or that of the Chelydroidaj 

 and of the Cinosternoida^, in which last three families ai'e preserved through life, 

 elements of form which recall the characteristic features of the Chelonii, but 

 which mostly disappear in the first years of life in the Emydoida?. In many 

 respects the form of the Emydoidaj approximates already that of the Testudinina, 

 to which the highest rank undoubtedly belongs, on account of the higher sym- 

 metry of the bod}^ 



This progressive series of the families of Testudinata, as far as it is based 

 upon their form, is not inferred simph* fi-om a vague estimate of the gradation 

 of these forms, as they appear in the adult, but rests upon a direct comparison 

 of the metamorphoses of the young, all of which undergo most remarkable 

 changes in their form. These changes are the more instructive, as they consti- 

 tute a connected series, when they are compared at certain stages of the growth 

 in dinVrent families, and yet the}' lead, in the end, in each faiiiih'. to the 

 development of a typical pattern characteristic of the family. Starting from a 

 common type at an early embrj^onic period, the form is gradually modified to 

 a certain degree, in one family, before it assumes its typical characters ; in another 

 family the same primitive type diverges in anotlu'r direction, and then assumes 



' Sec I'ail 1., Cliai). 2, Sect. 3, p. lo2-15i. 



