98 
TORTOISES AND TURTLES. 
approaching this conformation is to be met with among living representatives of 
the order. 
Generalised Certain extinet tortoises, such as Pleurosternum from the 
cheionians. Purbeck Oolite of Swanage, and Baena of the Eocene rocks of the 
United States, indicate the existence of an extremely generalised group of the 
order Amphichelydia, presenting many characters common to the existing 
S-necked and Side - necked 
groups, and which may have 
been the ancestral stock of 
both the latter. All have 
eleven bones in the plastron, 
owing to the presence of 
mesoplastrals, and an inter- 
gular shield, but the pelvis 
may or may not be connected 
w r ith the plastron. In the 
first of the genera named, 
the mesoplastral bones extend 
right across the shell to meet 
in the middle line, and one 
of the bones of the pelvis 
articulates to a smooth oval 
facet on the plastron. On 
the other hand, in the second 
genus, the mesoplastral bones 
are incomplete, as in the 
IMPERFECT CARAPACE OF WIDE-SHIELDED WEALDEN TORTOISE. . r 
existing greaved tortoises, 
and there is no union between the pelvis and the plastron. Since it is probable 
that the plastron of the Chelonians has originated from a system of abdominal 
ribs similar to those of the tuateras (Chapter VI.), it is interesting to notice that 
these generalised tortoises had a larger number of plastral elements than are to 
be found in the majority of the existing representatives of the order. 
The Soft-Tortoises. 
Family Trionychidje. 
The last group of the order comprises the soft river-tortoises, now confined to 
the warmer regions of Asia, Africa, and North America, but which, during the 
middle portion of the Tertiary period, appear to have been extremely abundant in 
the rivers of England and other parts of Europe. The whole of these tortoises are 
included in a single family which forms a group of equivalent value to the S-necked 
and Side-necked sections; and it is not a little remarkable that while in the 
greater part of their organisation they approximate to the former group, in certain 
features connected with the skull they come nearer to the latter. The most 
striking peculiarity of the soft-tortoises is to be found in the nature of their shells, 
