42 GUIDE TO THE FOSSIL REPTILES, AMPHIBIANS, FISHES. 
Well-preserved shells and other remains of Trionyx from the 
Upper Eocene of Hampshire are exhibited in Wall-case 18. 
Sub-order 2.—Cryptodira. 
WaH-cases Most of the known extinct Chelonia, like the majority of 
18 » 1 ^. existing tortoises and turtles, belong to the sub-order Crypto- 
1*/&S0S w 
to z. 
Fig. 40.—Lower view of the skeleton of the existing Logger-head Turtle 
(Thalassochelys caretta ); much reduced in size. 
dira (“ hidden-necked ”), in which the head is retracted by 
curvature of the neck in a vertical plane. The pelvis in 
these reptiles is not directly connected with the ventral 
armour or plastron. 
The ordinary marsh and land tortoises first occur in 
Eocene rocks, where modern kinds are associated with several 
extinct genera. Typical specimens are shown from the 
