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-lke 
foot of Testudo and the long and powerful flipper of Chelonia 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 vertebra, 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. 
