CHELONIA. 317 



"Reference has already been made (p. 1 78) to the impres- 

 sions in sandstones of triassic age in Dumfriesshire, of pro- 

 bably chelonian foot-prints. These have been finely illus- 

 trated in the great work by Sir William Jardine on the foot- 

 prints at Corncockle Muir. The earliest proof of chelonian 

 life which the writer has obtained, has been afforded by the 

 skull of the Chelone planiceps from the Portland stone ; and 

 by the carapace and plastron of the extinct and singularly- 

 modified genera Tretosternon and Pleurosternon* (fig. 109), from 

 Purbeck. In the first genus the plastron retains the central 

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

 interposed between the hyosternals (hs) and hyposternals 

 (ps). In the specimen figured, the plastron, and the under 

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

 of Pleurosternon emarginatum are shewn, f Emydian re- 

 mains, referred to the genera Hydroptelta and Aclielonia, have 

 been obtained from the lithographic (upper oolitic) slates at 

 Cirin. 



True marine turtles (Chelone Camperi, C. Benstedi, C. pul- 

 chriceps) have left their remains in cretaceous beds. \ The 

 emydian Protemys is from the greensand near Maidstone. § 

 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 whole 

 world, and all the eocene Chelones are extinct. One of them (C. 

 gigas, Ow.) attained unusual dimensions ; the skull, now in the 

 British Museum, measures upwards of a foot across its back 



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

 Limestones, 4to, 1853, Paleeontographical Society. 



-f- This fine Chelonite is in the possession of Wra. Cunnington, Esq. Devizes ; 

 and a similar specimen, from Swanage, Dorsetshire, has recently been acquired 

 for the British Museum. 



$ Owen, " Hist. Brit. Fossil Beptiles," pp. 155-168, pis. 41-56. 



§ Op. cit., p. 169, pi. 47. 



