394 AMERICAN TESTUDINATA. Part II. 



them more or less completely in old age. Carinated species also are more numer- 

 ous among the lower Amyda3 than among Testudinina ; all the Chelydroidse and 

 Cinosternoida3 are more or less carinated, especially in their younger age, and they 

 are inferior to the Emydoidte ; many of the most aquatic Emydoidaj are also cari- 

 nated, some through life, others only in the younger age; and we have already 

 seen that the aquatic species are inferior to the terrestrial ones, and that the 

 young Emydoida3 are more aquatic than the adults.^ 



From the few facts which I have already collected,^ I am convinced that much 

 valuable information could be obtained from a similar comparison of the changes 

 which our common Mammalia and Birds undergo in early life, and that the time 

 is not far distant when, in this way, the relative standing of the representatives of 

 every family will be determined with remarkable precision. The results to which I 

 have arrived by the study of the young Turtles will, I hope, stimulate other nat- 

 uralists to turn their attention also to tliis interesting subject. Happily the time 

 is coming when fewer new species are to be found, and, from want of materials 

 for their ordinary work of registering animals, with scanty or insufficient charac- 

 teristics, zoologists may be led to more important investigations. 



SECTION V 



GENERA OF TRIONYCHID^. 



It appears fi'om the statement of Dumeril and Bibron,^ that Schweigger was 

 the first to perceive the necessity of separating the soft-shelled Turtles as a dis- 

 tinct genus, which he called Amyda, in a paper presented by him to the Acad- 

 emy of Sciences in Paris, in 1809. Geoflroy, however, changed that name to 

 Trionyx,* which Schweigger himself adopted when he published his paper,^ as also 

 did all herpetologists afterwards. This genus was not further subdivided until 

 Wagler showed, in 1830, that it embraced species which exhibit marked structu- 

 ral differences, in the connection of the plastron and hmd legs, and in the ossi- 

 fication of the marginal rim. For those species which have bony plates along the 

 margin, and a wide hind lobe of the plastron, he retained the name of Trionjrx, 



^ Compare the note to p. 293. ' Erpet. gener. vol. 2, p. 464. 



^ See Agassiz, (L.,) Lake Superior. Boston, ^ Ann. du Mus. de Paris, vol. 14, 1809. 



1850, p. 191 ; also Twelve Lectures on Comparative ^ Prodromus Monographioe Clielouiorum ; Ko- 



Embrjology, p. 8 and 101. nigsberg. Archiv, 1812. 



