412 



AMERICAN TESTUDINATA. 



Part II. 



ing, or in attacking their prey and in defending themselves. The Chelydroids make 

 the same use of their tail when adult. The long tail of the young is there- 

 fore typical here, and not an embryonic feature, as it is in the Emydoids. The 

 Chelydroidas are mud Turtles ; they walk on the mud, or on the bottom of the 

 water, and, when put into the water, they instantly dive to the bottom. Nevertheless, 

 in this family, the feet are also better adapted for swimming in the early part of 

 life than later ; at least, the web between the toes is thinner, and thus the toes 

 more movable than in the adult. This is particularly obvious when comparing the 

 hind feet of the young Gypochelys with those of the adult; for in the latter they 

 are heavy, bulky, plantigrade, walking feet. 



Most of the characters which we have considered thus for are common to 

 the two American genera of Chelydroidte, Chelydra and Gypochelys. But there 

 are already features, in the young of the first year, which constitute generic 

 differences. This is particularly evident in the head and tail. The head of 

 the young Gypochelys exhibits already fully that wedge-shaped eagle bill, running 

 sharply down in front, by which it is so clearly distinguished from Chelydra when 

 advdt; Avhile, in the young Chelydra, the head is already much shorter, and the 

 jaws more rounded. Again, the tail distinguishes them also when young most 

 strikingly ; its lower surface, in Gypochelys, being covered with many small 

 more or less imbricated scales, just as in the Anguiformes among Lizards, while 

 in Chelydra, as in most Snakes, there run all along the under surface of the 

 tail, two rows of large scales. In Lizards and in Snakes, this amounts to a 

 family character, the scales of the tail being there of more importance than in 

 Turtles, in which we can only recognize generic diflerences in their peculiarities. 



The American members of this family are divided into two strongly marked 

 groups, one comprising the genus Gypochelys, the other the genus Chelydra. 

 These groups have clearly defined generic characters ; but it is a question, Avhether 

 some of their distinguishing characters have not a more than generic value. The 

 elements of form are in general the same in both ; but there are wide differences 

 in the forms of the head, which are, perhaps, such as to make each group a 

 sub-family.^ In Gypochelys every thing about the head is fitted to give the 



^ Whether the family of Chclydroiils contains two 

 sub-families or not, there can be no doubt that its 

 North American representatives bulong^ to two dis- 

 tinct genera. It will be easier to settle the question 

 of the sub-families after an opportunity has been had 

 to compare carefully the genus Platysterniim. It 

 may seem immaterial to ascertain this j>oint, when 

 it is considered that the whole family numbers only 



three genera. But, if the principles which I have ad- 

 vocated in the first part of this work are correct, it 

 will be found that Plafysternura will either be inter- 

 mediate between Chelydra and Gypochelys, in which 

 case the family would not be subdivided, or Pla- 

 tysternum will lean more towards one or the other 

 of the American genera, in which case it would at 

 once appear that it embraces two distinct sub-families. 



