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Tethys (Greek mythology the greatest)

in Greek mythology, the greatest of the sea deities, wife of Oceaetc., and about 3,000 daughters called nus, daughter of Uranus and Terra, and mother of the chief rivers of the universe, Nile, Peneus, Simois, Scamander, Oceanides. The name Tethys is said to periodic time, 1 day, 21 hours, 18 minsignify nurse. In astronomy, a Satellite utes, 25.7 seconds. In zoology a genus periodic time, 1 day, 21 hours, 18 min- utes, 25.7 seconds. In zoölogy a genus center of Saturn is 188,000 miles; its pressed; head covered by a broadly exof Tritoniade, with one species, from the Mediterranean, animal elliptical, depanded fringed disk with two conical ten- tacles; stomach simple. It attains a foot in length, and feeds on other mollusks and on small crustacea. an order of Cephalopoda, comprising the second family is classifications th three families; Nautildæ, Orthoceratidœ, and Ammonitide, though in some recent classificatio arms merged in the first. Animal creeping, protected by an external shell; head retractile within the mantle; eyes pedunous horny girdle; branchiæ four; funnel culated; mandibles calcareous; very numerous; body attached to shell by adductor muscles and by a continuformed by the union of two lobes which do not constitute a distinct tube. Shell external, in the form of an extremely elongated cone, either straight or vari- Cyc-Vol. IX TETRABRANCHIATA TETRAHEDRON ously folded or coiled, many-chambered, keeled and striated; first dorsal of nusiphuncled; the inner layers and septa merous feeble spines, and continuous with nacreous, the outer layers porcellanous.

In palæontology, they attained their maximum in the Palæozoic period, decreasing from that time onward, and being represented at the present by the single genus Nautilus. The Nautilidæ proper and Orthoceratidæ are pre-eminently Palæozoic, while the Ammonitide are almost exclusively Mesozoic.