Fossil fresh-water Tortoises.—The remains of fresh-water Tortoises, referable to the Emydidæ, occur in the Purbeck and Wealden strata (Owen’s Monograph, 1853, and Rep. Brit. Assoc. 1841); the resemblance of some of these to the Jurassic species from Soleure was noticed by Cuvier (Oss. Foss. vol. v.). Among the Chelonian remains of the Wealden, some of the most remarkable are the costal plates and other bones of a Tortoise, which in its essential characters is closely allied to the Trionyces,[689] but differs from the recent forms, in having possessed a dermal horny integument, formed of scutes of tortoise-shell. The chelonians of the genus Trionyx (so named from their having three claws) have the extremities of the ribs free, and not articulated to a border of marginal plates, and there are intervals between their costal plates even in the adult state. The external surface of the bones of the buckler is covered with granulations, or with little pits, for the attachment of the soft skin, the only integument with which these animals are invested; and, being destitute of horny scutes, their bones exhibit no furrows, as in the other genera. But the fossil rib-plates (see [Lign. 241]) have a shagreen-like or punctated surface, like the recent Trionyces, and at the same time bear the imprints of horny scutes; and, instead of being nearly of an equal width throughout their entire length, as in the existing species, have one extremity much wider than the other, as in the land-tortoises. From the slight degree of convexity of the ribs, it is evident that the carapace was much flattened, as in the Trionyx.[690] Except in having a defensive dermal integument, and agreeing in this respect with many of the Crocodilian reptiles, with which its bones are associated, the original must have closely resembled the existing predaceous fresh-water soft Turtles; and, doubtless, like those reptiles, inhabited the muddy beds of lakes and rivers, preying upon the eggs and young of the larger reptiles, and on the uniones and other fluviatile mollusca, whose shells are very commonly found imbedded with its remains.
[689] The relations of these peculiar remains to Trionyx were pointed out in Foss. S. D. 1822, p. 47.
[690] See Petrif. p. 157, &c.
Lign. 241. Tretosternon Bakewelli; [691] 1/3 nat. size.
Wealden. Tilgate Forest.
(Foss. Tilg. For. pl. vi. fig. 1.)
One of the costal plates.
[691] This Tortoise, with the sanction of Baron Cuvier, was described under the name Trionyx, in Foss. Tilg. For. 1827, p. 60, and its distinctive characters were pointed out. In Geol. S. E. 1833, p. 255, the specific name Bakewelli was proposed in honour of the late Robert Bakewell, Esq., whose excellent works have so greatly promoted the advancement of geology; a privilege to which, as the original discoverer of the species, and of its zoological relations, I was fairly entitled. But this name does not appear in the list of British Chelonians, either in Rep. Brit. Assoc. 1841, or in Mr. Morris’s Cat. Brit. Org. Rem. 1843. With a melancholy pleasure I now restore the name of my lamented friend, as a just, but very inadequate tribute of respect to his memory.