CHELONIA 281 



Order XII.— Chklonia. 



(Tortoises and Turtles.) 



Char. — Trunk-ribs broad, flat, suturally united, forming with 

 vertebrae and sternum an expanded thoracic abdominal 

 case, into which, as into a portable chamber, the head, 

 tail, and limbs can, usually, be withdrawn. No teeth : 

 external nostril single. 



Reference has already been made to the impressions in 

 sandstones of triassic age in Dumfriesshire, referred by Dr. 

 Duncan to tortoises. These impressions have been finely 

 illustrated in the great work by Sir William Jardine on the 

 footprints at Corncockle Muir. The earliest proof of chelonian 

 life which the writer has obtained has been afforded by the 

 skull of the Ghelone planic&ps, from the Portland stone ; and 

 by the carapace and plastron of the extinct and singularly- 

 modified emydian genera Tretostemon and Pleurosternon* (fig. 

 82). In the first genus the plastron retains the central 

 vacuity ; in the second genus an additional pair of bones is 

 interposed between the hyosternals (lis) and hyposternals (ps). 

 In the specimen figured (fig. 82), the plastron, and the under 

 surface of the marginal pieces (2 to 12) of the carapace, of 

 Pleurostemon cmarginatum are shown. This fine Chelonite is 

 now in the British Museum. 



True marine turtles (Ghelone Cumpcri, C. obovata, G. 



pulchriceps) have left their remains in cretaceous beds.t The 



emydian Protemi/s is from the greensand near Maidstone. t. 



The eocene tertiary deposits of Britain yield rich evidences of 



marine, estuary, and fresh-water tortoises. More species of 



true turtle have left their remains in the London clay at the 



mouth of the Thames than are now known to exist in the 



* Monograph of the Fossil Chelonian Reptiles of the Wealden and Purbeck 

 Limestones, -ito, 1853, Palseontographical Society. 



f Owen, "Hist. Brit. Fossil Reptiles," pp. 155-168, pis. 41-46. 

 | Op. eit., p. 169, pi. 47. 



