AgnostusArticle Free Pass Agnostus, genus of trilobites (an
extinct group of aquatic arthropods) found as fossils in rocks
of Early Cambrian to Late Ordovician age (those deposited from
540 to 438 million years ago). The agnostids were generally
small, with only two thoracic segments and a large tail segment.
Agnostus itself was only about 6 millimetres (0.25 inch) long
and lacked eyes. The similarity of the anterior and posterior
regions frequently makes the agnostids difficult to
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