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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.

Nidulite :: Nidulite (n.) A Silurian fossil, formerly supposed to consist of eggs..

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..

Quebec Group :: Quebec group () The middle of the three groups into which the rocks of the Canadian period have been divided in the American Lower Silurian system. See the Chart of Geology.

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

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

Silurian :: Silurian (a.) Of or pertaining 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..

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..

Cambrian :: Cambrian (a.) Of or pertaining to the lowest subdivision of the rocks of the Silurian or Molluscan age; -- sometimes described as inferior to the Silurian. It is named from its development in Cambria or Wales. See the Diagram under Geology.

Chaetetes :: Chaetetes (n.) A genus of fossil corals, common in the lower Silurian limestones..

Niagara Period :: Niagara period () A subdivision or the American Upper Silurian system, embracing the Medina, Clinton, and Niagara epoch. The rocks of the Niagara epoch, mostly limestones, are extensively distributed, and at Niagara Falls consist of about eighty feet of shale supporting a greater thickness of limestone, which is gradually undermined by the removal of the shale. See 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..

Silurian :: Silurian (n.) The Silurian age.

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..

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

Dalmania :: Dalmania (n.) A genus of trilobites, of many species, common in the Upper Silurian and Devonian rocks..

Utica :: Utica (a.) Of, pertaining to, or designating, a subdivision of the Trenton Period of the Lower Silurian, characterized in the State of New York by beds of shale..

Favosites :: Favosites (n.) A genus of fossil corals abundant in the Silurian and Devonian rocks, having polygonal cells with perforated walls..

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..

Medina Epoch :: Medina epoch () A subdivision of the Niagara period in the American upper Silurian, characterized by the formations known as the Oneida conglomerate, and the Medina sandstone. See the Chart of Geology..

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