258 AMERICAN TESTUDINATA. Part II. 



into fields, as it is also all over the neck and in all those other parts of the 

 trunk which are not covered by the shields of the back and the lower side. The 

 epidermis of the legs varies very much, from the thin layer of the Trionychidse, 

 in which it is only in some single places thickened into hard plates, to the horny, 

 scaly, or plated stiff coat of the massive feet of the sea and land Turtles, Chelonia 

 and Testudo, where there is very little or no motion of the different parts of 

 the legs. In the Chelonioida3 the epidermis of the last phalanges appears as a nail 

 only in the thumb, while in Sphargis there is not even a trace of a nail to be 

 found ; in the Trionychidaj it forms sharp, long, slim claws, in three fingers and in 

 three toes; in the aquatic Emyds (Nectemyds) there are similar nails in all the 

 fingers and in the toes. On the contrary, in the more terrestrial members of the 

 family of EmydoidaB, in Glyptemys insculpta, and still more in Cistudo, whose fingers 

 and toes are less movable and frequently used for walking on land, the claws 

 appear shorter and stouter, while in Testudo the whole coat of the fingers and 

 toes has become a hoof, almost as in Pachyderms, serving as in the latter to carry 

 the heavy load of the body. These epidermal formations in the legs and particu- 

 larly those in the last phalanges, in connection Avith the epidermal formations of 

 the jaws, are very important for the classification, as they indicate more clearly 

 than any other external organ the mode of life of the animal in all its relations 

 to the outer world. That the consideration of these parts leads really to natural 

 divisions is seen not only in Turtles, but more distinctly still in Birds and Mam- 

 malia ; and the system of Linnteus, founded upon such details, has assumed the 

 character of a natural combination in the classification of these two classes, though, 

 as he understood them, they still appear as artificial as his system of plants. 



The epidermis of the tail is mostly wrinkled or covered only by small scales, 

 thus allowing to this organ a great movability. In the fiimily of Chelydroidae 

 only do we find, along the top of their long, powerful tail, a row of hard 

 tubercles strengthening and protecting it as an organ of locomotion, and by no 

 means interfering with its movability. In some land Turtles and in the genus 

 Cinosternon, the end of the tail has a flat, rounded sheath, as in Testudo indica, 

 or it has a pointed nail-like or even crooked tip, as in Cinosternon, particularly 

 in the males. 



The most imjiortant features of the ei^idei'mis, and those most peculiar to Turtles, 

 are found in the back and the lower shield. It is scarcely developed in two 

 families, the Trionychidaj (soft-shell Turtles) and the Sphargididte, in which it fonns 

 only a thin continuous layer upon the coriuni, as in naked Batrachians, wliile it is 

 thick, horny, and divided into fields in all other Tcstudinata, that is to sa}^, in 

 all those Turtles in which the corium is entirely ossified. In the Trionychidae 

 and Sphargidte there lies always a thick layer of soft, unossified corium, under 



