CuAP. III. YOUNG TURTLES. 393 



be drawn in at all, or only partially, are inferior to the others, or exhibit what 

 may be called eiiibrz/oiiic characters.^ This is the case in the Chelonii, which have 

 always been considered as the lowest Testudinata, and, among Amydce, to some 

 extent in tlie Chel3^di'oid£e, which stand very low in their sub-order. In all 

 younger embryos the limbs arc paddles ; they remain paddles in the Chelonii, 

 whilst they are terminated by feet, with more or less distinct fingers, in the 

 AmydiB. We thus have here an additional evidence that the Chelonii are infe- 

 rior to the Araydtc. There is, however, a remarkable feature in the development 

 of the limbs in Chelonii : the paddles of the young sea Turtle, though identical 

 with those of the Amyda% differ from what they are in the adult age, and yet 

 they remain paddles. They exhibit, as it were, overgrown embryonic features, 

 such as characterize the types which I have called liiipemhryonk} 



The shield presents similar transformations. At first oblong, and narrower 

 behind than in front, it grows gradually broader, assuming even a circular form. 

 But the characters of the adult are already impressed upon the shield of the 

 Chelonii before it grows very wide ; so it is also with the Cinosternoidae and 

 Chelydroidae, while in Trionychidas the flat, roundish form in its fullest expansion 

 is that which the adult preserves. The Emydoidai have also reached that circu- 

 lar form at the time of hatching, but they afterwards grow again more elongated. 

 The question thus arises, Is there a retrograde development in the Emj-doidiu, or 

 not ? For my part, I am satisfied that it is not the case. Considering the differ- 

 ence of the elongated form of the Emydoidte, in which the hind end is generally 

 the broadest, whilst in the elongated shield of the embrj-o this is the reverse, 

 and considering further the closer relation of the Emydoidai and Testudinina, in 

 which latter the two ends of the body balance one another so evenly, I believe 

 that the elongation of the Emydoidaa, subsequent to their circular outline, marks 

 a real progress. I consider, therefore, the later widening of the Chelonii, as observed 

 in the adult, as a progressive development, which is attained only late in life in 

 that family ; so that it might be said, that, in this respect, the Chelonii do not even 

 reach in old age the form to which the Trionychidaj and Emydoida^ attain at the 

 time of hatching, and at which the Trionychidse stop, whilst the Emydoida^ take 

 another start in a higher direction, to approximate the form prevailing in the adult 

 Testudinina. A knowledge of the early embryonic changes of the Testudinina is 

 still wanting to cany out fully these comparisons. 



I am inclined to consider, further, the presence of keels along the back as 

 characters of inferiority, considering the prominence of these keels in the lowest 

 Chelonii, the Sphargidida;, and their presence in young Chelonioidtv, which lose 



> Comp. Part I., Cli. 1, Sect. 25, p. 112 to IIG. » See Part I., Cb. I, Sect. 25, p. 116. 



50 



