Page:Collier's New Encyclopedia v. 09.djvu/370

LEFT TETRABRANCHIATA 318 TETRAHEDRON ously folded or coiled, many-chambered, siphuncled; the inner layers and septa nacreous, the outer layers porcellanous. In palaeontology, they attained their maximum in the Palaeozoic period, de- creasing from that time onward, and be- ing represented at the present by the sin- gle genus Nautilus. The Nautilidoe prop- er and Orthoceratidce are pre-eminently Palaeozoic, while the Amnionitidoe are al- most exclusively Mesozoic. TETRACERA, in botany, a genus of Delimeoe, owing its scientific name to the fact that its four capsules are recurved like horns. Shrubs or small trees, often climbing, with alternate, stalked, feath- er-nerved, naked leaves, often rough above, and panicled or racemose inflores- cence. A decoction of T. hreyyiiana and T. oblongata is given in Brazil in swell- ing of the legs. T. tigarea is diapho- retic, diuretic, and antisyphilitic. TETRACHORDON. in music, an in- strument similar in appearance to a cot- tage pianoforte, and like it played by finger-board, but the tone, instead of be- ing produced by striking, is obtained by means of a cylinder of india-rubber charged with resin, kept in motion by a pedal, variety of tone being gained by the depth of pressure on the keys by the fin- gers. It is called the tetrachordon from an idea that its sounds are similar to those produced by a string quartet. The instrument is constructed also with self- acting machinery. Milton used the word as the title of one of his treatises on mar- riage, occasioned by his disagreement with his wife, Mary Powell. He ex- plained the word in the sub-title: "Ex- positions upon the Four Chief Places of Scripture which treat of Marriage." TETRADYMITE, a rhombohedral mineral found sometimes in crystals, but more frequently granular, massive, or fo- liated, often with auriferous ores; luster, bright metallic; color, pale steel-gray; in thin laminae, flexible. Composition : Some- what variable, but consists principally of bismuth and tellurium. Dana divides as follows: (a) Free from sulphur, with formula Bi2Tej; (6) Sulphurous, with formula, BUiysTe+YsS),, and (c) Sele- niferous. Also the same as joseite and wehrlite. TETRAGONIACE^, a small order of plants, alliance Ficoidales. No petals, and several consolidated carpels. They are succulent, leaved, herbaceous plants, or small shrubs, chiefly natives of New Zealand. TETRAGONURUS, in ichthyology, a genus of Atherinidm, with a single spe- cies. Body sub-elongate, scales strongly keeled and striated; first dorsal of nu- merous feeble spines, and continuous with the second. It is a rare fish, more fre- quently met with in the Mediterranean than in the Atlantic. Nothing is known of its habits, but as, when young, it ac- companies the Medusae, it must be re- garded as a pelagic form. At a later period of its existence it probably de- scends to greater depths, coming to the surface only at night. It attains a length of about 18 inches. TETRAGRAPTTJS, in palaeontology, a genus of Graptoliiidse from the Skiddaw and Quebec groups (Lower Silurian). The polypary consists of four simple mono-prionidian branches, springing from a central non-celluliferous connecting process, which bifurcates at each end. The celluliferous branches do not sub- divide, and the base may be enveloped in a peculiar horny disk. TETRAGYNIA, an order of plants in Linnaeus' artificial system. It consisted of plants having four pistils. The classes Tetx'andria, Pentandria, Hexandria, Hep- tandria, Octandria, and Polyandria, have each an order Tetragynia. TETRAHEDRITE, a name given to a group of minerals having considerable diversity in composition, but presenting the same general formula. Named from the prevailing tetrahedral habit of its crystals. Crystallization isometric, fre- quently twinned; luster, metallic; color and streak, steel-gray to iron-black; opaque; fracture, sub-conchoidal, uneven; brittle. Composition, essentially a sul- phantimonite of copper, with the formu- la 4CuS-fSb2S3; but in consequence of part of the copper being frequently re- placed by iron, zinc, silver, mercury, and occasionally cobalt, and part of the anti- mony by arsenic or bismuth, the general formula is usually written as 4(Cu, Fe, Zn, Ag, Hg)-f (Sb, As, Bi)=S3. Dana di- vides tetrahedrites as follows: (1) An antimonial series. (2) An arsenio-anti- monial series. (3) A bismuthic-arsenio- antimonial and an arsenical series, in which the antimony is entirely replaced by arsenic. The varieties are: (1) Ordi- nary, containing very little or no silver. (2) Argentiferous = freibergite. (3) Mercurif erous = schwa tzite, spaniolite, and hermesite. (4) Platiniferous. Field- ite, aphthonite and polytelite are sub- species. An abundant ore in many parts of the world, sometimes, where rich in silver, mined for that metal only. TETRAHEDRON, or T E T R A E - DRON, in geometry, a polyhedron bound- ed by four triangles. If the middle points of the faces be properly joined, two and two, the lines joining them are