MODIFICATION IN TURTLES. 21 



To permit these different modes of flexion the mechanical arrangements must 

 be different. In the Cryptodires and the tnonychids 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 vertebrae 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 Baena 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 Baemdae, 

 so far as known, and possibly in Glyptops, the prepubic process was strongly 

 ossified. In the Emydidae and the Testudinidae the ischio-pubic bar is ossified; but 

 in the Cheloniidae 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- 

 nychidae 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, 



