THE AQUATIC VERTEBRATES 1027 



!and. The soft-shelled turtles usually lay their eggs in sandy beaches, 

 sometimes in harder banks near the water. The painted turtles 

 and snapping turtles dig holes more remote from the water's edge. 

 The musk turtles lay their eggs in muck, in decaying stumps or 

 logs, or accumulations of decaying weeds on the margins of lakes. 



No one would seriously doubt that the mammahan and rep- 

 tilian faunas of fresh waters have both been derived from terres- 

 trial ancestors. The adjustment to water conditions consists largely 

 in an adaptation of the limbs and tail to swimming and diving. 

 Both are organs primarily used for land progression. Further 

 adaptations in reptiles, such as the utiHzation of the oxygen in 

 the water by the turtles, are much more rare, and found only in 

 extreme adaptations to an aquatic sojourn. 



The paths of turtles may readily be seen among fields of Chara 

 in shallow water. A painted, geographic, or a musk turtle may 

 be seen basking in the sun on the surface, the neck curved up, the 

 nose out of water. If disturbed it dives into the Chara and soon 

 disappears in one of its innumerable paths. A curious commen- 

 salism is reported between the soft-shelled turtle and the black 

 bass. The bass is said to follow the turtle, which, nosing about 

 under rocks and in crannies scares out some of the crayfishes and 

 other denizens of such places. These are then easily captured by 

 the attending black bass. There is a peculiar correlation between 

 the disposition of turtles and the degree of their armature. The soft- 

 shelled turtle is the least protected by bony plates. Next in order 

 comes the snapping turtle, with only a cross-shaped, ventral plate, 

 most of the ventral surface being open to attack. This is followed 

 by the musk turtle, the painted and the geographic turtle, Bland- 

 ing's turtle and finally the box turtle. The highest degree of pro- 

 tection is found in the terrestrial box turtle, whose plastron is 

 hinged and can be closed in front and behind. Correlated with 

 the defective armature in the soft-shelled turtle we find the extreme 

 of pugnacity. A soft-shelled turtle will snap and bite on suspicion 

 from the time it is half way out of its shell. The disposition of 

 the snapping turtle, with exposed ventral surface, is proverbial. 

 The musk turtle will bite, as anyone who has collected their eggs 

 can testify. On the other hand, the well-protected painted, geo- 



