CiiAP. ni. GENERA OF CHELYDROID.E. 411 



The youn^ of the I'amily of Cheljclroida) exhibit new features, difierent from 

 those which we have noticed before in sea Turtles, in Eniydoida?, and in Trio- 

 nychidiv. When hatched, they start, like the TrionychidiB and Emydoidaa, with a 

 circular body ; but their body is relatively much higher than that of the Trio- 

 nychidaj and Emydoidae, and flattens out with age. The circular form grows 

 first more and more oval, then oblong, in Gypochelys, (PI. 5, fig. 23-27,) by a 

 straightening of the lateral margin ; while in Chelydra (PI. 4, fig. 13-16, and PI. 

 5, fig. 18 and 19) an oval circumference is j^ermanent throughout life. The orna- 

 mental bass-relief which ajjpears upon the surfiice is not less peculiar in Chelydroida\ 

 In Gypochelys it exists all over the body ; in Chelydra particularly on the upper 

 shield, where the corium rises in the form of larger and smaller warts and ridges. 

 Besides smaller warts, which are spread irregularly all over the body in Gypo- 

 chelys, and over the shield in Chelydra, we see in both genera three rows of lon- 

 gitudinal I'idges formed by the UKidian and the two costal plates of the back. These 

 ridges are homologous to the three longitudinal rows of the young Thalassochelys 

 and of the genus Chelys. The homology of Gypochelys with the latter genus is 

 even carried so far, that, in the adults, the horny plates as well as the corre- 

 sponding bony shields, when only seen from above, could hardly be distinguished. 

 Even that curious twisting, characteristic of the lateral ridges, is the same in both 

 cases, and the sutures between the costal plates run through them in exactly the 

 same places. We see here a homology of forms connected with the greatest dis- 

 crepancy of structure ; for the true skeleton of Chelys, taken as a whole, is so 

 difierent from that of the Chelydroida% as to justify fully their separation as dis- 

 tinct families.^ Beyond these three ridges, we find, in the young Gypochelj's, two 

 more ridges on the top of the marginal plates. These are wanting in the young 

 and in the adult Chelydra, and nearly so in the adult Gj-pochelys. Moreover, 

 in the adult Chelydra, the three median ridges fade also more and more with 

 advancing age, and we have seen large adult specimens which were entirely smooth. 

 The lateral and posterior marginal plates of the young of this family are narrower 

 outwardly than where they are attached to the costal plates. This causes the 

 circumference of the posterior half of the trunk to appear deeply scalloped in 

 Gypochelys, but less so in Chelydra, where these indentations disappear more and 

 more with advancing age. At the first sight, the tail would seem, on account 

 of its great size, to be an organ adapted for similar functions as in young Emy- 

 doids, in which we found it also relatively very long ; but upon closer examina- 

 tion we may soon* be satisfied that the round, strong tail of the Chelydroids. 

 though very long, is not a rudder as in young Emydoids, but a support in walk- 



* See the family characters of ChclydroiJie and Cheljoids, p. 335-3-lG. 



