262 LEAVES FROM THE 



conformity with this dispensation, the anterior extremity 

 of the upper bill in the majority of species exhibits a 

 large notch, and on each side of it a sufficiently strong 

 tooth, reminding the observer of the beak of the higher 

 raptorial birds. 



In some of this group. Nature, which, in the chelo- 

 nian forms that we have already noticed, had contented 

 herself with a lid either before or behind, carries out 

 what may be termed the box principle, by making, as 

 in the genus Cistudo, a moveable lid both before and 

 behind. In this subgenus a cartilage attaches the wide 

 oval plastron to the buckler. This cartilage is moveable 

 both before and behind, turning on the same transversal 

 mesial hinge, and, at the will of the animal, presenting 

 nothing but a well-closed box to the prying eyes of the 

 enemy. In Kinosternon, also, the oval sternum is move- 

 able before and behind on a fixed piece ; but in Stauro- 

 typus, the thick cruciform sternum is moveable in front 

 only. In others, again, Platysternon and JEmysaura, 

 for example, the plastron is immovable. 



The Potamians, or true river-tortoises, whose species 

 have been confounded under the name of Trionyx, have 

 amono" them some which grow to a considerable size. 

 To say nothing of one which was kept by Pennant, and 

 weighed twenty pounds, seventy pounds have been 

 stated as the weight attained by certain individuals. In- 

 habiting the streams and rivers, or great lakes of the 

 warmer regions of the earth, their habits are generally 

 similar. Swimming with much ease either upon or be- 

 neath the surface of the water, they pursue young croco- 

 diles, other reptiles, and fishes, which their agility enables 

 them to make their prey. They are also said to be great 

 destroyers of the eggs of the crocodiles, especially in the 

 Nile and the Ganges. The angler baits his hook for 

 them with small fishes or other living bait, unless his 

 skill enables him so to play a dead or artificial one as to 



