Sea turtle Sea turtle Sea turtles (superfamily Chelonioidea), sometimes called marine turtles, are reptiles of the order Testudines and of the suborder cryptodira. The seven existing species of sea turtles are: the green, loggerhead, Kemp's ridley, olive ridley, hawksbill, flatback, and leatherback. The majority of a sea turtle's body is protected by its shell. The turtle's shell is divided into two sections: the carapace (the dorsal portion) and the plastron (the ventral portion). The shell is made up of smaller plates called scutes. The leatherback is the only sea turtle that does not have a hard shell. Instead, it bears a mosaic
History of manifolds and varieties History of manifolds and varieties The study of manifolds combines many important areas of mathematics: it generalizes concepts such as curves and surfaces as well as ideas from linear algebra and topology. Certain special classes of manifolds also have additional algebraic structure; they may behave like groups, for instance. In that case, they are called Lie Groups. Alternatively, they may be described by polynomial equations, in which case they are called algebraic varieties, and if they additionally carry a group structure, they are called algebraic groups. The term "manifold" comes from German "Mannigfaltigkeit," by Riemann. In English, "manifold" refers to