TRIONYX EMYS. 257 
the principles of their construction, with the 
conditions by which existing Turtles are fitted 
for their marine abode, that Cuvier was at once 
enabled to pronounce these fossil species to have 
been indubitably inhabitants of the sea.* 
The genera Trionyx and Emys, present their 
fossil species in the Wealden freshwater forma- 
tions of the Secondary series ; and still more 
abundantly in the Tertiary lacustrine deposits ; 
all these appear to have lived and died, under 
circumstances analogous to those which attend 
their cognate species in the lakes and rivers of 
the present tropics. They have also been found 
* Plate 25', Fig. 4, represents a Turtle from the slate of 
Claris : it is shewn to have been marine by the unequal elon- 
gation of the toes in the anterior paddle ; because, in freshwater 
Tortoises, all the toes are nearly equal, and of moderate length ; 
and in land Tortoises, they are also nearly equal, and short ; but 
in all marine species they are very long, and the central toe of 
the anterior paddle, is by much the longest of all. The accord- 
ance with this latter condition in the specimen before us, is at 
once apparent; and both in this respect and in general structure, 
it approaches very nearly to living genera. This figure is copied 
from Vol. 5, Pt. 2, Tab. 14, /. 4, of the Oss. Foss. of Cuvier. 
M. Agassizhas favoured me with the following details respecting 
important parts which are imperfectly represented in the drawing 
from which Cuvier's eng^ravino; was taken. '* The ribs show 
evidently that it is nearly connected with the genera Chelonia 
and Sphargis, but referrible to no known species ; the fingers of 
the left fore paddle aie five in number ; the two exterior are the 
shortest, and have each three articulations ; and the three in- 
ternal fingers, of which the middle one is the longest, have 
each four articulations, as in the existing genera, Chelonia and 
Sphargis." 
GEOL. s 
