genus of trilobites (an extinct group of aquatic arthropods) found as fossils in Silurian and Devonian rocks (between 359,000,000 and 444,000,000 years old) in Europe and North America. Phacops is a common and easily recognizable form, with its rounded rather than angular outline, globose head region, and large compound eyes. A common form is the species Phacops rana.