420 AMERICAN TESTUDINATA. Part II. 



The characteristic peculiarities of the eggs of the Cinosternoid£e have abeady 

 been mentioned (p. 350). Those of Thyrosternum pennsylvanicum are represented 

 PL 7, fig. 1-6 ; those of Ozotheca odorata, fig. 7-9. 



In the young Ozotheca odorata, and still more in the young Thyrosternum penn- 

 sylvanicum, the characteristic features and forms of the ftimily are already so 

 fully developed during the first year, that we can hardly point out any change 

 in their forms, from young to adult. This holds good, not only for the general 

 proportions and outlines of the upper and lower shield, the feet, and the tail, 

 but also for the scales. In the adult Emydoidae, as well as in the Cinoster- 

 noidse, the median scales of the carapace are generally narrower than the costal 

 ones. This is already fully the case in all Cinosternoidae, at the time of hatch- 

 ing; while in Emydoidaj exactly the reverse obtains. (See p. 293, note 1, for 

 a description of the young Chrysemys, and also PI. 4 and 5.) In Thyrosternum, 

 Platythyra and Ozotheca, the median scales of the back are, from the first year, 

 not broader than long; while in Emyds they are at least twice, and often three 

 times as broad during the first year as later in life. This peculiarity no 

 doubt contributes to give them an oldish appearance from the beginning. There 

 is another feature which makes the young Cinosternoidaj look old : the rounded 

 margin of the carapace and its steep curve behind, which are already fully 

 marked, during the first year, in Thyrosternum and Platythyra. The sharper 

 margin and the less prominent curve, which characterize Ozotheca in contradis- 

 tinction to Cinosternon, are likewise strongly marked in the young Ozotheca, 

 even more strongly than in the adult. Moreover, the tail has the same propor- 

 tions from the first year to adult age. As the CinosternoidiB are walking Tur- 

 tles, living in mud like the Chelydroidae, they do not need a long and high 

 tail as a rudder. Notwithstanding this early development of the prominent feat- 

 tures of these Turtles, we have to point out one interesting change in the 

 Ozothecoids. When young, they are all high and carinated. These characters 

 are brought out most fully in Goniochelys triquetra ; while Ozotheca odorata, 

 which, when young, shows the same height and the same keel on the back, 

 grows more and more flat in course of time. 



The family of CinosternoidiB is composed of two well defined groups. In 

 one, the true Cinosternoids, the plastron is large, and imderlies nearly the whole 

 body ; the bridges which connect it with the carapace are long, and the first 

 and fourth pairs of its bony plates are broad and rounded, and connected with 

 the intermediate pairs by very flexible hinges. Thus the spaces around the free 

 edges of the plastron are small, and, when the animal withdraws and raises the 

 ends of the plastron, the soft parts of the body are almost entirely protected. 

 In the other group, the Ozothecoids, the plastron is smaller; the bridges are shorter, 



