388 AMERICAN TESTUDINATA. Part II. 



very unequal sizes. The plates of the sternum grow broader as the animal grows 

 older, just the opposite of what we see in the Amydae. This is, however, much more 

 extensively the case with the two median rows than with the lateral rows of the 

 bridge, which latter are nearly as broad in the hatching Caouana as the median 

 ones; while in the adult, their transverse diameter is hardly more than one third 

 of that of the median ones. The connection of this change of the form of the 

 plates with the change of the whole shape of the trunk, as described in this sec- 

 tion for the Chelonioidag, and above (p. 294) for the Emydoida3, is self-evident. The 

 sculpture of the plates is exceedingly fine in the hatching Th. Caouana. This sculp- 

 ture is preserved in some land Turtles and some Emydoidge throughout hfe, but 

 soon fades away in the sea Turtles. As this sculpture of the plates rests merely 

 in the epidermal plates, it is not to be confounded with the wart-like excrescences 

 which we meet with in the hatching Chelydroidre and Cinosternoidse. The latter 

 consist in real thickenings of the coi'ium, which ossify on a very large scale in 

 Gypochelys, and are homologous to the rows of tubercles in Caouana which have 

 been described above. 



The tail of the young sea Turtles is exceedingly short; not any longer, in pro- 

 portion to their size, than in the adult. This, again, is different from what we see in 

 hatching Amydse, where the tail of the young is so remarkably long ; in the Emy- 

 doidi\3, nearly as long as the whole carapace. If we attempt to give an explanation 

 for this strange discrepancy, we are led to the conclusion that it must be owing 

 to the circumstance, that, as in young Emydoidce all the four feet serve as paddles 

 and the tail acts as a rudder, while in sea Turtles the front feet only are pad- 

 dle-! and the hind feet serve as rudder, the Chelonioida) do not need such a strong 

 rudder tail as the young Emydoida^, which have no rudder but the tail, their hind 

 feet being paddles. In relation to this use of the hind feet as rudders in sea 

 Turtles, we refer to PI. 6, fig. 13, 15, and 16, which show the green Turtle in a 

 swimming attitude. The hind feet of Thalassochelys Caouana, when hatching, are 

 very broad, and the front feet also are broader and much longer in comparison 

 than in the adult. The claws of the thumb and the first finger are long and 

 strong, while in the adult they fade nearly entirely away. 



Having thus described the young Thalassochelys Caouana as the most acces- 

 sible representative of the family of Chelonioida^ at the time of hatching, and com- 

 pared it with the adult as we have before described the changes which the Amydse 

 undergo from the time of their birth to adult age, exemplifying these metamor- 

 phoses in our common Chrysemys picta, we may now proceed to compare the 

 earlier changes which Turtles undergo in the egg, with a view of ascertaining 

 how the differences exhibited by the two sub-orders of Testudinata are to be 

 understood. 



