Somewhat approaching the Giant Tortoises in size are the Afri can T. calcarata and the South American T. tabulate but the re maining species of the genus are relatively small ; T. elegans, the Indian Star Tortoise, is black with yellow streaks radiating from the centre of each shield and the South African T. geometrica has a similar pattern; in Europe there are 2 species, T. hermanni of S. Europe and Asia Minor and T. marginate of Greece, the species commonly imported into England being T. graeca of northern Africa and Asia Minor. The N. American Gopher Tortoises (T. Polyphemus) are burrowing creatures inhabiting dry, sandy country and constructing their own burrows, but the East African T. tornieri is cryptozoic in rocky country and hides in crevices.
The true Turtles ; the three common species are the Hawksbill (Eretmochelys imbricate) the Green (Chelonia mydas) and the Loggerhead (Caretta caretta). Of these, the first mentioned may be distinguished by its smaller size, the overlap ping horny plates of the back and the hooked beak; it occurs in all tropical seas and is the source of the "tortoise-shell" of com merce. The horny shields of the carapace are stripped off by heat and when heated in oil or by steam they can be welded together to form slabs thick enough for manufacturing purposes. The Green Turtle is the species used so much in the manufacture of turtle soup; it is a larger species than the Hawksbill attaining a length of close on 4 feet and the horny plates of the back do not overlap but meet edge to edge. All these turtles have the same gen eral habits and distribution ; the Green Turtle only is partially or entirely herbivorous, feeding on Zostera the marine grass, the others preferring fish or molluscs. All lay their eggs in pits dug by the females on sandy beaches which are visited at night and it is then that large numbers are captured; the eggs, too, are edible so that in some regions the turtles are becoming scarce.
Pleurodira.
Shell covered with horny shields; limbs not paddle shaped.
A. Neck completely retractile into the shell.
Pelomedusidae.
B. Neck long, not hidden by the shell when retracted. Chelydidae.
Shell covered with soft skin; limbs paddle-shaped. Carettochelydidae. Pelomedusidae, fresh-water dwellers in Africa and South America ; Pelusios of Tropical Africa and Madagascar has the front lobe of the plastron hinged but Pelomedusa of the same regions lacks this character. Podocnemis similar to Pelomedusa inhabits the rivers of tropical S. America and reaches a consider able size.
Chelydidae, the "Snake-Necked Turtles" of the fresh-waters of S. America, Australia and Papua. They are all long-necked creatures with the typical depressed shells of aquatic tortoises, the many species are superficially very similar, the S. American Matamata (Chelys'fimbriata) alone showing any peculiarities.
Carettochelydidae, the single, rare Papuan species of this family, Carettochelys insculpta, superficially resembles the marine turtles in its paddle-shaped limbs, but the absence of horny plates is distinctive and suggests affinities with the next group.
This contains but a single family, the Triony chidae. In this family the bony carapace and plastron are much reduced in size and the whole creature is very flat, almost circular in bodily outline and entirely covered with soft skin; the digits are broadly webbed, only the three inner having claws, the jaws are hidden by fleshy lips and the snout is prolonged into a short, soft proboscis. All the members of the group are dwellers in the mud of slow-running rivers, lakes and ponds and are car nivorous; some species show what appears to be dimorphism connected with their diet, some individuals which appear to be fish eaters having sharp-edged jaws and others having broad, crushing surfaces which suggest a diet of molluscs. The six genera of the family are distributed throughout Africa, Asia and North America and are superficially very similar. Trionyx, the most widely dis tributed genus, has species in regions inhabited by the family.
W. P.)