REPTILES. 293 



the extremities flattened, arid enveloped in a horny integu- 

 ment, to form oars for swimming. The anterior pair are 

 nearly double the length of the posterior pair. (Pig. 196.) 



FIG. 198. Portion of the Carapace of a Fossil Turtle, from St. Helen's. 

 (Size of the original carapace sixteen inches long, and nine wide.) 



The head is proportionally large, and cannot be retracted 

 within the carapace : as a compensation, it is protected by an 

 accessory bony shield, formed by an exogenous growth from 

 the frontals, parietals, post-frontals, and mastoids, and not 

 by the introduction of any new bones into the architecture 

 of the cranium. The remains of this family have been found 

 in the muschelkalk, in the weal den, the lower stage of the 

 cretaceous period at G-laris, and the upper stage of the same 

 formation at Maastricht. In the marine eocene tertiary beds 

 of Sheppey, Bracklesham, Bognor, -and Harwich, are found 

 perfect skulls, carapaces, and plastrons of chelone.* Eleven 

 species are described and figured in the valuable monograph 

 referred to ; it is a remarkable fact that the clays of the Isle 

 of Sheppey contain a greater number of species of CJielone 

 than are known to exist in our modern seas. Zoologists 

 know five species of existing Chelone, and only two of these, 

 CJiel. midas, and CJiel. caouanna, frequent the same locality. 

 "It is obvious, therefore," says Professor Owen, "that the 

 ancient ocean of the eocene epoch was much less sparingly 

 inhabited by turtles, and that these presented a greater 

 variety of specific modifications than are known in the seas 

 of the warmer latitudes of the present day. 



* Pictorial Atlas, plate Ixix. 



