Phacops is a genus of trilobites found as fossils in Silurian and Devonian rocks (between 345,000,000 and 430,000,000 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. Phacops rana is one of the most common trilobite found in the Middle Devonian of North America. Strangely enough its close relatives are found 4,000 miles away in Northwest Africa. A great deal of evidence suggests that Northwest Africa was very close to eastern North America during the Devonian Period (between 408 - 360 million years ago). It is thought that a collision with Africa was the origin of the Appalachian Mountains, as they exist today.

ORDER PHACOPIDA

Cephalon: typically proparian (Phacopina and Cheirurina) or gonatoparian (Calymenina), preglabellar field often very short or absent, 4 or fewer pairs of glabellar furrows (these sometimes fused); eyes, when present, schizochroal (Phacopina) or holochroal (Cheirurina and Calymenina); with rostral plates (Calymenina and Cheirurina) or without (some Phacopina); hypostome conterminant (all suborders) to impendent (some Phacopina).
Thorax: 8 – 19 segments, sometimes distinctly furrowed, axis sometimes broad (e.g., Homalonotidae).
Pygidium: typically micropygous (most Calymenina and Phacopina), but variable (e.g., subisopygous in Dalmanitoidea and Acastoidea), may be lobed or spiny (e.g., Cheirurina, some Dalmanitoidea, Acastoidea), or smooth-margined, with round or subtriangular outline (e.g., Calymenina, Phacopoidea).
Other: All suborders share similar ontogeny (particularly in the protaspides) that in particular link the Calymenina to other Phacopida.
Occurrence: Lower Ordovician (Tremadoc) to Upper Devonian (Famennian)
Suborders: Calymenina, Phacopina, Cheirurina.

SUBORDER PHACOPINA

Cephalon: proparian sutures (sometimes fused), schizochroal eyes; glabella expands forwards; librigena typically yoked as single piece; hypostome conterminant to impendent, some with no rostral plate; some with genal spines.
Thorax: 10 to (typically) 11 segments, pleurae furrowed, articulating facets distinct, rounded, angular, or spinose tips.
Pygidium: typically smaller than cephalon (but subisopygous in Dalmanitoidea and Acastoidea), smooth or spinose.
Superfamilies: Phacopoidea, Dalmanitoidea, Acastoidea

SUPERFAMILY PHACOPOIDEA

Cephalon: generally with strongly divergent axial furrows (glabella greatly expanding anteriorly), anterior glabellar lobes fused into single anterior tri-composite lobe, frontal area generally lacking (obliterated by large glabella), eyes (when present) typically anterior; genal angle typically rounded, without genal spines (exceptions among Pterygometopidae), vincular furrow generally present.
Thorax: variable, but often with rounded tips.
Pygidium: typically micropygous (Phacopidae) to subisopygous (some Pterygometopidae), not spinose.
Families: Phacopidae, Pterygometopidae
Genera: Ductina, Kainops, Phacops, Pterygometopus, Reedops.