726 SIMOUOIOES SHOR SIMGUOIBIN IOS 
impossible to consider more than a few of the genera. There 
seems to be a division of the group in very early times into land- 
living and water-living forms. The first of these gave rise to the 
common swamp and land turtles of today, and the second group 
to the sea turtles, the Prnnata. 
The second of these groups may be considered first ; Dermo- 
chelys is the most specialized of the genera. This is a gigantic 
form still living in all of the oceans, though little known from its 
almost exclusively pelagic habits. The carapace has entirely 
disappeared and is replaced by a tough leathery skin that has 
won for them the name of ‘‘Trunk-backs or leather-turtles.”” The 
plastron is less completely removed but there is a large space in the 
center, between the bones called the fontanelle. The fore limbs 
are developed as flippers, the anterior pair being much the 
longest. 
The more common members of the same division are the 
Loggerhead and Green turtles. In these the carapace and 
plastron are less reduced, but both show large vacuities that 
indicate the steps by which the condition in Dermochelys was 
produced. 
Protostega is the earliest of the definitely known forms. It is 
from the Upper Cretaceous (Niobrara) of Kansas. Similar in 
many respects to the recent Dermochelys, it still shows many 
primitive characters that indicate its separate position. The 
carapace was not ossified, but the proximal ends of the ribs show 
lateral expansions that would, if carried a little farther, produce 
the condition found in the Chelonia (the most primitive of the 
living sea turtles). The genus was one of the largest of the order. 
One specimen measured nearly or quite seven feet in length. 
Protosphargis, from the Upper Cretaceous of Italy, Zosphargis, 
from the London Clay (Eocene), and Psephophorous, from the 
Miocene of Germany, are all closely related to Protostega and 
show successive steps toward the condition of Dermochelys. The 
great geographical extent of these forms seems to indicate that 
they possessed the same roving and pelagic habits that char- 
acterize their nearest living relative. 
