2 FOSSIL TURTLES OF NORTH AMERICA. 
ON THE OSTEOLOGY OF THE TUR TEES: 
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 1s represented by enough of its skeleton, and 
nearly every bone of a turtle is so characteristic that its neloman nature is evident. 
It is generally supposed that the most characteristic thing about a turtle is the 
fies PRR ERAOT of the bones of the body walls into a sort ie box, into which the 
animal can withdraw its head, legs, and tail. However, no one would mistake the 
relationships of the leatherback (eat: helys), 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, relativ ely 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 deseahed 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 Emydidz, a central el 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 (nw. p). Behind this comes a series of 8 neurals (n. 1, 1. 2, etc.). 
The last of these is followed by two suprapygals (spy, 1, 2), and the hindermost of 
these is succeeded by the pygal (py), forming the last oi the median series. The 
8 neurals are connected with the neural arches of the dorsal vertebrae, of which 
they appear to be mere expansions. The nuchal, the suprapygals, and the pygal 
are not connected with the vertebre. 
On each side of the row of neurals and articulating with them are 8 costal plates, 
or -costals-(e-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 
