Trilobite Terminolgy

     Amid the flurry of phrases that fill this trilobite website, there are probably a few words that are less than instantly familiar to anyone who is not an inveterate invertebrate aficionado. And since our intent is to bring the fascinating world of these ancient arthropods out of the classroom and laboratory and into the mainstream, we figured it might be beneficial to present an abbreviated “glossary” of key trilobite-related terms. While trying not to emulate either a textbook or a science journal, we believe knowing a little bit more about these occasionally tongue-twisting morphological terms may enhance your experience as you peruse this site:

Appendages: The antennae, limbs and walking legs that all trilobites possessed… and which, on rare occasion, appear as fossilized remnants.
Axis: The central lobe of a trilobite's three-lobed body design.
Cephalon: The head of a trilobite, housing the eyes, among other important morphological features.
Compound eyes: The multi-lensed eyes that often are the most pronounced feature of a trilobite cephalon.
Doublure: A cephalic or pygidial rim that stretches under the ventral side of the trilobite.
Dorsal: The top (back) side of a trilobite.
Enrollment: A feature that allowed some trilobites to assume a ball-like shape in order to protect their vulnerable ventral underside.
Exoskeleton: The outer shell, or external skeleton, of the trilobite. The most-often fossilized remnant of these Paleozoic creatures.
Free cheeks (Librigena): The segments of a trilobites' cephalic shell, surrounding on either side of the glabella, outside of the facial sutures, which can be jettisoned during molting.
Fixed cheeks (Fixigena): The segments of cephalic shell surrounding the glabella, inside of the facial sutures, which are not jettisoned during molting.
Genal spine: Any spine emanating from the cheek area of the cephalon.
Glabella: The mid-section of the trilobite cephalon  -- often convex in shape.
Holochroal: A type of trilobite compound eye where all the lenses are in direct contact and are covered by a single calcite surface.
Hypostome: The trilobite's mouthplate, located on the ventral side of the cephalon.
Isopygous: When the trilobites' head and tail are roughly the same size.
Lagerstatte: A location where the fossil preservation is of particular scientific importance.
Opistothorax: A long worm-like extension of the thorax that appears on certain Lower Cambrian trilobites. May indicate the trilobites' worm-like antecedents.
Paleozoic: The nearly 300 million year-old era that stretches from the Lower Cambrian through the Permian, and which neatly bookends the rise and eventual fall of trilobites.
Prosopon: Finely detailed terracing or structures that appear on the trilobites' outer shell.
Pygidium: The tail-end of a trilobite.
Schizochroal: A compound eye featuring separate and distinct lenses.
Spinose: A feature where prominent spines appear on the trilobite exoskeleton.
Suture: Breaks in the trilobite carapace which facilitate the molting process. 
Telson: A pronounced tail spine emanating from the pygidium.
Thorax: The middle part of the trilobite's anatomy, often featuring between 8 and 15 flexible segments.
Ventral: The underside of a trilobite where muscle attachments scars (and occasional soft body parts) may be seen.