8 FOSSIL TURTLES OF NORTH AMERICA. 



the first row of the tarsus. The fibula is slender, with the broader end downward, 

 articulating with t,he same bone as does the tibia. The bone of the tarsus just 

 mentioned is the only one present in the upper row, and is regarded as representing 

 three bones that theoretically belong in the upper row, besides the centrale. In the 

 lower row of tarsal bones there are five present, all articulating above with the 

 large bone of the upper row and distally each with one of the metatarsals. The 

 fifth of these bones is peculiarly expanded in the turtles. 



The 5 metatarsals are rather elongated in Graptemys and Trachemys, as are, 

 too, the phalanges. In the first digit there are 2 phalanges; in each of the others, 3. 

 All the terminal phalanges are invested in horny claws, except the fifth. 



THE CHELONIID/E. THE SEA-TURTLES. 



Some of the salient features of the Cheloniidae will now be described. These 

 constitute a family of the Cryptodira, a superfamily to which also the Emydidae 

 belong. The Cheloniidae are greatly different in some respects from the Emydidae. 

 The shell is composed of the same elements, but many are less completely developt. 

 In general, the border of the carapace is excavated in front for the neck; and on 

 each side of this for free movement of the fore limbs. Behind, it is pointed, so 

 that in form the carapace is somewhat heart-shaped. The costal plates fall short 

 of reaching the distal ends of the ribs, as a result of which open spaces, or fontanels, 

 are left between the costals and the peripherals. Usually none of the costal plates 

 come into contact with the peripherals. The ribs of the costals, however, reach 

 these bones and each enters a pit in a corresponding peripheral. In the loggerhead 

 (Caretta caretta], for example, rib-ends enter peripherals 3 to 8 inclusive and 

 peripherals 10 and 11. In some forms, as Lepidochelys, there are supernumerary 

 bones interposed among the neurals, so that the number of these seems to be as 

 high as 14. 



The carapace is furnisht with scutes similar to those of the Emydidae. In 

 Caretta and Lepidochelys there is a supernumerary costal scute in front of the 

 normal first. 



Altho the plastron of these turtles is composed of the same elements as that of 

 the Emydidae, some of these are greatly modified. The hyoplastra and the hypoplas- 

 tra are not suturally connected with the peripherals of the bridge; nor do the 

 plastral bones just named come into contact with their fellows at the midline, so 

 that the space between the bones of the right and of the left sides is occupied by a 

 great fontanel. On each side there is another fontanel inclosed by the bridge 

 peripherals, the outer end of the hyoplastron, and that of the hypoplastron. The 

 inner and the outer borders of the two sets of bones last mentioned send out a 

 number of digitations into the fontanels. The epiplastra are saber-shaped bones 

 which join each other and the entoplastron at the midline, without jagged sutures; 

 and their distal ends are applied to the outer border of the corresponding hyo- 

 plastrals. The xiphiplastra are narrow, curved bones that approach each other 

 at their distal ends. 



Carapaces and plastra similar to those of the Cheloniidae are found among the 

 Thalassemydidas, and to these the reader is referred for illustrations. 



The neck of the Cheloniidae is much shorter than that of the Emydidae and the 

 head can hardly be retracted within the shell. In the loggerhead the neck is but 

 little more than half as long as the series of dorsal vertebrae. The eighth cervical 

 articulates by a synovial surface with the inferior side of the nuchal bone. The 

 series of caudal vertebrae is short. 



