REPTILIA 241 



DIVISION 2. PLEURODIRA 



The Pleurodira play the same role in the southern hemisphere that 

 is played by the Testudinidse in the northern regions. They are less 

 diversified, however, than the northern tortoises in that they are all 

 aquatic. They differ from our tortoises mainly in that the neck, in- 

 stead of being withdrawn within the carapace between the shoulders, 

 is bent laterally and tucked under the edge of the shell on one side. 

 The pelvic girdle, unlike that of our tortoises, is fused to the tail and 

 to the carapace. 



The genus Chelodina will serve as an example of these southern 

 tortoises. The carapace is much like that of Chrysemys, but the plas- 

 tron has a novel feature in the form of a small median scute, the inter- 

 plastral, which is believed to be a vestige of an ancestral row of scutes 

 that has been lost by most turtles. They are good swimmers and feed 

 exclusively upon aquatic animals such as frogs and water insects. 

 The long neck undulates from side to side like that of a snake. When 

 basking they tuck the head away under the shell in the manner de- 

 scribed. There seems to be no striking difference between these tor- 

 toises and our own with respect to breeding and nest-making habits. 

 The Snake-Necked Turtle, Hydromedusa maximiliani (Fig. 134, A) is 

 another familiar example of this sub-order. 



TRIONYCHID.E (SOFT-SHELLED TORTOISES) 



The distinguishing character of these tortoises is their lack of the 

 scaly or chitinous armature. They also lack parts of the bony arma- 

 ture possessed by other groups. All over the body there is a reduction 

 of the scaly elements; on the feet the scales are reduced to soft folds of 

 skin. The "soft shells" are, however, not to be pitied for their de- 

 fenseless state, for they make up for their loss by their greatly in- 

 creased intelligence and rapidity of locomotion. Aspidonectes spinifer 

 (Fig. 134, B) is the common "soft shell" of the Mississippi basin 

 and is familiar to most residents of that region. Of all our tor- 

 toises they are the most exclusively aquatic, coming inshore only 

 for nesting purposes, and seldom basking except upon floating logs 

 and upon low river banks very close to the water's edge. They 

 always turn around after crawling out of the water, so as to have 

 the head turned toward the water, ready to scramble into the river 

 again at the slighest suggestion of danger. They have need to 



