2 FOSSIL TURTLES OF NORTH AMERICA. 



ON THE OSTEOLOGY OF THE TURTLES. 



The turtles, or tortoises, form an order of the great class of reptiles; and they 

 probably constitute the best defined of all the orders of that class, no member of 

 the group having yet been discovered about whose chelonian affinities there need 

 be any dispute, in case the animal is represented by enough of its skeleton, and 

 nearly every bone of a turtle is so characteristic that its chelonian nature is evident. 



It is generally supposed that the most characteristic thing about a turtle is the 

 transformation of the bones of the body walls into a sort of box, into which the 

 animal can withdraw its head, legs, and tail. However, no one would mistake the 

 relationships of the leatherback (Dermochelys), altho little more than a crocodile 

 or a cayman does he possess a box into which he can withdraw his exposed members. 

 It is rather the broad, relatively short body of these animals, the small head on a 

 more or less elongated neck, and the toothless, horn-covered jaws that stamp these 

 animals as turtles. 



Of turtles existing at the present day there are somewhat more than 200 species. 

 Dr. George A. Boulenger, in his Catalogue of Chelonians (1889), estimated the 

 number at 201 species, and a number of others have been described since that 

 time. They inhabit all the continents and all parts of the continents that are not 

 too cold nor too dry for them. Many islands, even some, as the Galapagos, far 

 removed from continental bodies of land, possess or have possest their species. 

 All the warmer seas are visited by pelagic turtles, of which there are at least 5 

 species. Even dry and hot deserts may support their peculiar forms of these 

 animals. The geographical distribution of the living species will be discust on 

 another page. The geological history extends back into the middle Triassic; and, 

 with little doubt, turtles existed in the Permian. 



THE EMYDID/E. THE TERRAPINS. 



For the information of the reader a general description will here be given of the 

 osteology of members of the principal groups of turtles. We shall begin with the 

 shell of one of the Emydida?, a central family of the order. This belongs to the 

 geographic turtle [Graptemys geographica), an inhabitant of the lakes and larger 

 streams of the Mississippi Valley. The shell reaches a length of at least 225 mm. 

 and is only moderately archt. In this case, as in the great majority of the members 

 of the order, it forms a box into which the head, legs, and tail can be retracted. 



The shell consists of two parts, an upper, the carapace (fig. 1 ), and a lower, the 

 plastron (fig. 2). On each side, between the fore and the hind leg, the two portions 

 are joined by the bridge. The carapace is composed of 50 bones, each of which 

 is articulated by jagged sutures with the adjoining bones. In front and behind 

 the bridges, the outer ring of bones projects freely like the eaves of a roof. Twelve 

 of the bones of the carapace are arranged in a row along the midline. In front is 

 the nuchal bone (mi. p). Behind this comes a series of 8 neurals (. 1, 11. 2, etc.). 

 The last of these is followed by two suprapygals (spy, 1, 2), and the hindermost of 

 these is succeeded by the pygal (/>y), forming the last of the median series. The 

 8 neurals are connected with the neural arches of the dorsal vertebra?, of which 

 they appear to be mere expansions. The nuchal, the suprapygals, and the pygal 

 are not connected with the vertebra?. 



On each side of the row of neurals and articulating with them are 8 costal plates, 

 or costals (c. p, I, 2, etc.). These appear to be only expansions of the ribs; and 

 study of the embryology shows that they are connected with the ribs from the 

 earliest appearance of the bone. Nevertheless, there are reasons for believing that 



