386 AMERICAN TESTUDINATA. Part II. 



A large species of this family has been found by Professor Francis S. Holmes, 

 of Charleston, in the tertiary deposits of South Carolina. Other specimens, from 

 the miocene of New Jersey, have been described by Dr. J. Leidy under the name 

 of Chelone grandaeva, and others still, from the green sand, imder the name of 

 Chelone ornata;^ but, whether they belong to the genus Chelonia as now limited, 

 or to Thalassochelys, or to Eretmochelys, is not yet ascertained. 



SECTION IV. 



COMPARISON OF THE GROWTH OF THE CHELONII WITH THAT OP THE AMYDiE. 



The investigation of the general form of young Emydoidie, and a minute com- 

 parison with the adults,^ has led to the result, that all Emydoidre exhibit, when 

 hatching, a circular form, which grows more and more elliptical with advanc- 

 ing age. This law of morphological development does not hold good for sea 

 Turtles. On the contrary, they are much longer in proportion to their width, 

 when hatching, and then grow gradually broader. The upper shield of Thalasso- 

 chelys Caouana, when hatching, has a longitudinal diameter of 0",045, and a trans- 

 verse diameter of O^jOSo ; a fortnight after, the relation is 0'°,046 to 0",038; 

 after twenty-one days, 0'",050 to 0'",042 ; and in the half grown, 0",275 to 0'",250. 

 This clearly shows a change from a longer to a broader form, just the reverse of 

 what is observed in the Amydae. How is this to be understood ? Is the develop- 

 ment of the form just the opposite in these two sub-orders, or is it, perhaps, that 

 the Amydaj have already run through the form of the Chelonioida3 while in the 

 egg, and appear now round when hatching, to grow again more and more ellij^ti- 

 cal? The inference from this last view of the case would be, that the Cheloni- 

 oidte only reach in their highest perfection, namely, in the adult state, (Thalasso- 

 chelys Caouana,) the form which the Amydre exhibit when hatching. This view 

 is at least sustained by the facts which lie before us ; but further comparisons, 

 particularly of young Sphargididaa, must show whether this is the law. But, before 

 considering more fully the evidence thus fiir collected upon this point, let us 

 examine more minutely the pecuUarities which our young Thalassochelys Caouana 

 exhibits, at the time it is hatched. 



As in the Amydce, the head of the Th. Caouana, when hatching, is exceedingly 

 large. The horn by which the eggshell is broken is a solid excrescence of the 



1 Proc. Acad. Nat. Sc. Phila., vol. 5, p. 329, vol. 8, p. 303. = See above, Chap. 1, Sect. 4, p. 290 to 295. 



