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he latest period of the Silurian age. Oriskany period, a subdivision of the American Paleozoic system intermediate or translational in character between the Silurian and Devonian ages. See Chart of Geology.

An extinct genus of Brachiopoda, abundant in the Paleozoic rocks.

An extinct genus of Paleozoic Cephalopoda, having a long, straight, conical shell. The interior is divided into numerous chambers by transverse septa.

An extinct order of sea urchins found in the Paleozoic rocks. They had more than twenty vertical rows of plates. Called also Palæechini. [Written also Palæechinoidea.]

A suborder of Crinoidea found chiefly in the Paleozoic rocks.

ite, or in the respective positions 1 and 4; 2 and 5; or 3 and 6, as paraxylene; paroxybenzoic acid. Cf. Ortho-, and Meta-. Also used adjectively.

A genus of extinct Paleozoic brachiopods, often very abundant in the Upper Silurian. Pentamerus limestone (Geol.), a Silurian limestone composed largely of the shells of Pentamerus.

next following the Carboniferous, and regarded as closing the Carboniferous age and Paleozoic era. -- n.

nces. Phthalic acid (Chem.), a white crystalline substance, C6H4.(CO2H)2, analogous to benzoic acid, and employed in the brilliant dyestuffs called the phthaleins.

An extinct order of Mesozoic marine reptiles including the genera Plesiosaurus, and allied forms; -- called also Sauropterygia.

les, having a very long neck, a small head, and paddles for swimming. It lived in the Mesozoic age.

An extinct order of flying reptiles of the Mesozoic age; the pterodactyls; -- called also Pterodactyli, and Ornithosauria.

the class of reptiles attained its highest expansion; -- called also the Secondary or Mezozoic age.

Pertining to, or of the same horizon as, certain Mesozoic strata of the Rhetian Alps. These strata are regarded as closing the Triassic period. See the Chart of Geology.

cluding numerous species, many of them of large size. They are characteristic of the Paleozoic formations. The radiating septs, when present, are usually in multiples of four. See Cyathophylloid.

A trade name for benzoic sulphinide. [Written also saccharin.] C7H5NO3S.

as an antiseptic, and in its salts in the treatment of rheumatism. Called also hydroxybenzoic acid.

s. Secondary accent. See the Note under Accent, n., 1. -- Secondary age. (Geol.) The Mesozoic age, or age before the Tertiary. See Mesozoic, and Note under Age, n., 8. -- Secondary alcohol (Chem.), any one of a series of alcohols which contain the radical CH.OH united with two hydrocarbon radicals. On oxidation the s…

m (also used adjectively) denoting the presence of silicon or its compounds; as, silicobenzoic, silicofluoride, etc.

ning to the country of the ancient Silures; -- a term applied to the earliest of the Paleozoic eras, and also to the strata of the era, because most plainly developed in that country.