22 FOSSIL TURTLES OF NORTH AMERICA. 



never coming on land except to deposit their eggs, the fore limbs have become 

 transformed into definite flippers with the fingers elongated and all bound together 

 in a common mass of skin and muscles. Only one or two claws remain. The 

 phalanges and carpals are considerably flattened, as is also the humerus. The 

 latter also becomes straighter and the insertions of the deltoid muscle descend on 

 the shaft. While these changes progrest the hinder limb became relatively smaller 

 and fitted for steering the animal. The extreme of these modifications is to be 

 seen in the limbs of the leatherback, Dermochelys. Between the short club-like 

 foot of Testudo and the long and powerful flipper of Cheloma and Dermochelys there 

 is a vast interval. 



Amid all the changes that have occurred in the turtles certain fundamental 

 structures have remained unaffected. The jaws have always retained their horny 

 covering. The quadrate has remained fixt. The cervical vertebrae have kept 

 unchanged their number, 8, and the dorsals their original number, 10. All four 

 of the limbs have persisted and all the segments of each. 



A consideration of the changes which turtles have undergone and a comparison 

 of these with the modifications suffered by other groups of reptiles lead to the 

 conclusion that no other order of these animals, except the Squamata, can display 

 such a variety of structures. The Squamata, embracing the mosasaurs, the lizards, 

 the chameleons, and snakes, have undoubtedly, in adaptation to different modes 

 of life, diverged in more directions and gone farther than have the turtles. The skull 

 has become modified in more ways; the vertebrae, at least as regards the number in 

 the column, have varied more; the limbs have undergone more varied adaptations 

 for walking, for climbing, for grasping, for swimming, for leaping; and in the whole 

 group of snakes and in many lizards the limbs have wholly disappeared. To these 

 denizens of the earth, swarming since probably the Triassic, the turtles must yield in 

 variety of form and structure and habits, but probably to no other order of reptiles. 



