392 



AMERICAN TESTUDINATA. 



Part II. 



the representatives of different fiimilies resemble one another more in proportion 

 as they are younger. But the pecuharities which distinguish them most promi- 

 nently do not make their appearance at the same time. Features which belong 

 to a later stage of growth in one family become distinct in other famihes at a 

 much earlier period of life.^ Some stop at one point, while others undergo fur- 

 ther changes. Yet, the order in which these changes take place is so uniform, 

 that it may furnish the means of determining the relative standing of these ani- 

 mals, as soon as it is admitted that the characters which distinguish the earliest 

 stages of growth are inferior to those of the mature development. 



The great size of the head and neck is a remarkable feature in all the young 

 Testudinata, in no one of which are these parts retractile. The proportions are 

 greatly changed afterwards, and the head and neck become retractile in the 

 Amyda?. I take it, therefore, that large-headed Turtles, the head of which cannot 



cessive appearance of the characters peculiar to 

 groups of a different importance may be in different 

 branches of the animal kingdom. In Insects, for in- 

 stance, the class cliaracters, — the tracliea' and articu- 

 lated legs, — appear always before the ordinal charac- 

 ters, the wings ; the family characters, — the form, — 

 are also fully defined before the ordinal characters 

 appear, etc. How different from what we have seen 

 in the Testudinata! 



1 A glance at PI. 1 to G will show to what ex- 

 tent the young representatives of some families dif- 

 fer in form from the adult, and how early others 

 acquire their family characters. All the figures of 

 these plates represent young Turtles in their natural 

 size at the time of hatching, or as nearly at that 

 time as I could obtain them. Yet neither the Che- 

 LONioiD^, {Thalassochelys Caouana, PI. 6, fig. 13- 

 IG,) nor the Trionychid^, (Aspidonectes spinifer, 

 PI. G, fig. 1 and 2 ; Aspidonectes Emoryi, PI. G, fig. 4 

 and 5 ; Platypeltis ferox, PI. G, fig. 3 ; Amyda mutica, 

 PI. G, fig. 6 and 7,) nor the Cheltdroid^, ( Chelydra 

 serpentina, PI. 4, fig. 13-1 G, and PI. 5, fig. 18 and 

 19 ; Gypochelys Temminckii, PI. 5, fig. 23-27,) nor 

 the CiNOSTEUNOiD*, {Ozotheca odorata, PI. 4, fig. 1- 

 6 ; Ozotheca tristycha, PI. 5, fig. 20-22 ; Cinosternum 

 pennsylvanicum, PI. 4, fig. 7-12, and PI. 5, fig. IG and 

 17 ; Cinosternum fiavescens, PI. 5, fig. 12-15 ; Cinos- 

 ternum sonoriense, PI. 5, fig. 8-11,) exhibit marked 

 differences in their form from the adults ; or, what 



amounts to the same, their family characters are fully 

 developed, not only at the time of hatching, but even 

 long before. The Emydoidje, on the contrary, — 

 (such as Ptychemys concinna, PL 1, fig. 13, PI. 2, fig. 4— 

 G ; Ptychemys moUliensis, PI. 3, fig. 14-lG ; Ptychemys 

 riigosa, PL 2G, fig. 1-3 ; Trachemys elegans, PI. 3, fig. 

 9-11 ; Trachemys scabra, PL 2, fig. 13-15 ; Graptemys 

 geographica, PL 2, fig. 7-9 ; Graptemys LeSueurii, 

 PL 2, fig. 10-12, and PL 5, fig. 5-7 ; Malacoclemmys 

 paluslris, PL 1, fig. 10-12 ; Chrysemys picta, PL 1, fig. 

 1-5, and PL 3, fig. 4 ; Chrysemys marginata, PL 1, fig. 

 G, and PL 5, fig. 1-4 ; Chrysemys oregonensis, PL 3, 

 fig. 1-3 ; and Chrysemys Bellii, PL G, fig. 8 and 9 ; 

 Deirochelys reticulata, PL 1, fig. 14-16, and PL 2, fig. 

 1-3 ; Emys Meleagris, PL 4, fig. 20-22 ; Nanemys 

 guttata, PL 1, fig. 7-9 ; Aciinemys marmorata, PL 3, fig. 

 5-8; Cistudo virginea, PL 4, fig. 17-19; and Cistudo 

 ornata, PL 3, fig. 12 and 13,) — have almost perfectly 

 circular outlines, and exhibit in no way the slightest 

 tendency to the more or less elongated form of the 

 adult, with the exception perhaps of Malacoclemmys 

 palustris, and Deirochelys reticulata, which are 

 slightly oval ; so that, at the time of hatcliing, no 

 Emydoid has assumed the form characteristic of that 

 family. Xerohates Berlandieri, PL 5, fig. 17-19, the 

 only young representative of the family of Testudi- 

 nina which I had an opportunity of examining, shows 

 that these Turtles also are obicular before they as- 

 sume their final, characteristic form. 



