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Cliff Limestone :: Cliff limestone () A series of limestone strata found in Ohio and farther west, presenting bluffs along the rivers and valleys, formerly supposed to be of one formation, but now known to be partly Silurian and partly Devonian..

Graptolite :: Graptolite (n.) One of numerous species of slender and delicate fossils, of the genus Graptolites and allied genera, found in the Silurian rocks. They belong to an extinct group (Graptolithina) supposed to be hydroids..

Primordial :: Primordial (a.) Of or pertaining to the lowest beds of the Silurian age, corresponding to the Acadian and Potsdam periods in American geology. It is called also Cambrian, and by many geologists is separated from the Silurian..

Atrypa :: Atrypa (n.) A extinct genus of Branchiopoda, very common in Silurian limestones..

Salina Period :: Salina period () The period in which the American Upper Silurian system, containing the brine-producing rocks of central New York, was formed. See the Chart of Geology..

Trinucleus :: Trinucleus (n.) A genus of Lower Silurian trilobites in which the glabella and cheeks form three rounded elevations on the head.

Cincinnati Epoch :: Cincinnati epoch () An epoch at the close of the American lower Silurian system. The rocks are well developed near Cincinnati, Ohio. The group includes the Hudson River and Lorraine shales of New York..

Wenlock Group :: Wenlock group () The middle subdivision of the Upper Silurian in Great Britain; -- so named from the typical locality in Shropshire.

Trenton Period :: Trenton period () A subdivision in the lower Silurian system of America; -- so named from Trenton Falls, in New York. The rocks are mostly limestones, and the period is divided into the Trenton, Utica, and Cincinnati epochs. See the Chart of Geology..

Paleozoic :: Paleozoic (a.) Of or pertaining to, or designating, the older division of geological time during which life is known to have existed, including the Silurian, Devonian, and Carboniferous ages, and also to the life or rocks of those ages. See Chart of Geology..

Archaean :: Archaean (n.) The earliest period in geological period, extending up to the Lower Silurian. It includes an Azoic age, previous to the appearance of life, and an Eozoic age, including the earliest forms of life..

Phacops :: Phacops (n.) A genus of trilobites found in the Silurian and Devonian formations. Phacops bufo is one of the most common species.

Ordovician :: Ordovician (a.) Of or pertaining to a division of the Silurian formation, corresponding in general to the Lower Silurian of most authors, exclusive of the Cambrian..

Tilestone :: Tilestone (n.) A kind of laminated shale or sandstone belonging to some of the layers of the Upper Silurian.

Ludlow Group :: Ludlow group () A subdivision of the British Upper Silurian lying below the Old Red Sandstone; -- so named from the Ludlow, in Western England. See the Chart of Geology..

Llandeilo Group :: Llandeilo group () A series of strata in the lower Silurian formations of Great Britain; -- so named from Llandeilo in Southern Wales. See Chart of Geology.

Taconic :: Taconic (a.) Designating, or pertaining to, the series of rocks forming the Taconic mountains in Western New England. They were once supposed to be older than the Cambrian, but later proved to belong to the Lower Silurian and Cambrian..

Trilobite :: Trilobite (n.) Any one of numerous species of extinct arthropods belonging to the order Trilobita. Trilobites were very common in the Silurian and Devonian periods, but became extinct at the close of the Paleozoic. So named from the three lobes usually seen on each segment..

Pentamerus :: Pentamerus (n.) A genus of extinct Paleozoic brachiopods, often very abundant in the Upper Silurian..

Delthyris :: Delthyris (n.) A name formerly given to certain Silurian brachiopod shells of the genus Spirifer.

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