CHELONIA.
(Tortoises and Tartlets.) C'har.—Truuk-ribs broad, fiat, 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an expanded thoracic abdominal case, into which, as into a portable chamber, the head, tail, and limbs can, usually, be withdrawn.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 Chelone planiceps, from the Portland stone ; and by the carapace and plastron of the extinct and singularly modified emydian genera Tretosternon and (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 (he) 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 Pleurosternon emarginatum are shown. This fine Chelonite is now in the British Museum.
True marine turtles (Chelone Camperi, C. obovata, C. Indchriceps) have left their remains in cretaceous beds.t The emydian Protemys 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 whole world ; and all the eocene Chelanes 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 The estuary genus Trionyx (soft Pleurosternon emarginatum (Purbeck).
turtle) is represented by many beautiful species in the upper eocene at Hordwell ;t the fresh-water genera Emys and Platemys by as many species, both at Sheppy and Hordwell. In the pliocene of cEningen remains of a species of Clotlydra have been discovered ; this generic form is now confined to America.
Remains of land-tortoises (Testudo, Brong.) indicate several extinct species iu the miocene and pliocene formations of continental Europe. Strata of like age in the Sewalik Hills have revealed the carapace of a tortoise 20 feet in length ; it is called by its discoverers, Cautley and Falconer, Colossochelys atlas. The same locality has also afforded the interesting evidence of a species of Emys (E. tectum, Gray) having con tinued to exist from the (probably miocene) period of the Sivatherium to the present day.