MODIFICATION IN TURTLES. 21 
To permit these different modes of flexion the mechanical arrangements must 
be different. In the Cryptodires and the trionychids there are ginglymoid joints 
at the base of the neck, facilitating bending in a perpendicular plane. The neural 
arches are low, with the zygapophyses wide apart, favoring motion in the vertical 
plane, restricting it in the horizontal. In the Pleurodires the arches are high, the 
zygapophyses close together, often confluent, and at least one end of most of the 
centra semiglobular, arrangements aiding motion in a horizontal plane. 
It is doubtful whether there is another group of vertebrates that possesses so 
many modifications of the cervical vertebra as are found in the turtles. 
‘There has occurred a considerable amount of modification in the pelvis of these 
reptiles. As shown by Glyptops, of the Jurassic, and Baéna and Chisternon, of the 
Bridger, the pelvis was not originally suturally joined to the shell. In both the 
Cryptodires and the trionychids the pelvis has retained its original freedom. In 
the Pleurodires the ilia effected, before the close of the Cretaceous, strong sutural 
connections with the hindermost costal plates; while the ischia and the pubes 
became closely sutured with the xiphiplastra. 
In the Amphichelydia the ischia and the pubes are joined along the midline by 
a bar of bone, thus defining right and left ischio-pubic foramina. In the Baénide, 
so far as known, and possibly in Glyptops, the prepubic process was strongly 
ossifed. In the Emydidz and the Testudinidz the ischio-pubic bar is ossified; but 
in the Cheloniidz and the Trionychoidea this region is wholly cartilaginous. ‘That 
the primitive condition of the ischio-pubic bar and of the prepubic process in the 
Amphibia and the early Reptilia was cartilaginous we can not doubt. That these 
should be ossified in the Amphichelydia is remarkable. It suggests that the carti- 
laginous condition in so many living tortoises may be due to degeneration. 
The limbs of most swamp-loving turtles present primitive conditions of the 
reptilian limb, both with respect to their composition and their disposition. The 
segments of the limbs are mostly flext in one plane, and this plane stands more or 
less at right angles with the axis of the body. The apex of the angle at the elbow 
is directed forward, rather than backward as in the mammals, and the ulna and the 
radius do not cross. There have occurred few unions of bones and these are con- 
fined to the carpus and the tarsus. No bones have suffered important reductions 
or modifications. In the limbs of swamp-inhabiting turtles there have been few 
changes since Jurassic times; and, since the Amphichelydia, the Cryptodira, and 
the Pleurodira possess similar limbs, it is evident that the primitive turtles possest 
limbs not greatly different. 
From this simple type of limb there has been divergence in two directions; one 
to adapt the animal for life on the land, the other for habitual life in the water. 
The highest expression of the former adaptation is perhaps to be found in the limbs 
of species of Testudo. In these the principal modification in the proximal bones 
of the limbs is the drawing downward and toward each other of the tuberosities of 
the humerus. Most important is the shortening suffered by the bones of the digits 
and the reduction of the number of phalanges in each to no more than two. The 
fifth hinder digit may become vestigial. The result of these changes is the pro- 
duction of a short foot resembling that of an elephant and adapted for travel over 
rough and hard ground. 
To fit the animal for habitual life in the water the anterior limb tends to be 
converted into a flipper. First of all, the fingers become elongated to support a 
broad web. Usually the number of phalanges remains unaffected. In the Trio- 
nychidz some of the digits are much elongated and the phalanges are more numerous 
than the normal, and two of the claws have disappeared. In the sea-turtles, 
