Ankylosed:	no facial suture evident in ankylosed forms
Axial ring:	central portion of thoracic segment or a segment of the pygidium
Axis:	central lobe running the length of the trilobite body
Cephalon:	head (anterior) portion of the body
Cranidium:	central portion of cephalon bounded laterally by facial suture
Doublure:	reflexed continuation of dorsal exoskeleton onto ventral surface, typically as a ventral cephalic and/or pygidial rim
Enrolment:	the ability of a trilobite to roll themselves up so that only the hard exoskeleton is exposed. Usually involves tucking the pygidium beneath the cephalon
Eye lobe:	containing the palpebral lobe and visual surface of the eye
Eye ridge:	raised band extending from anterior end of eye lobe to anteriolateral angle of glabella
Facial Suture:	linear break in cephalon along which the free cheeks may separate from the cranidium during molting. Types include: opisthoparian, proparian, gonatoparian, marginal and ankylosed
Free cheek:	part of cephalon lateral to the cranidium which separates along the facial suture during molting
Genal angle:	posterolateral corner of cephalon
Genal spine:	posteriorly directed extension of border and doublure at genal angle, forming pointed projection
Glabella:	axial portion of cephalon, typically convex and lobed
Glabellar furrow:	bilaterally symmetrical pairs of narrow grooves on the surface of the glabella. The glabellar furrows are situated between glabellar lobes
Gonatoparian:	facial suture with posterior section reaching cephalic margin at genal angle
Holochroal:	compound eye consisting of numerous adjoining lenses covered by a continuous cornea	Compound or Holochroal eye
Hypostome:	a large plate with a central "blister" is attached to the rear edge of the anterior cephalic doublure, typically the anterior edge of the hypostome lies directly under the anterior edge of the glabella
Marginal brim:	an extensive, (usually) pitted area developed around the anteriolateral cephalic border. Only found in the suborders Harpina and Trinucleina
Marginal suture:	suture traversing marginal band of cephalon, as in Harpetidae. Not usually visible
Median lobe:	central area of the glabella
Occipital ring:	axial region of most posterior segment of cephalon, (considered part of the glabella)
Occipital spine:	single or multiple spine/s projecting from the occipital ring
Opisthoparian:	facial suture with posterior section cutting posterior margin of cephalon
Palpebral lobe:	portion of fixed cheek (cranidium) associated with the eye
Pleura:	lateral portion of thoracic segment
Pleural furrow:	groove in surface of thoracic pleura or pleural region of pygidium
Pleural rib:	portion of pygidial pleural region bounded by two successive pleural furrows
Preglabellar field:	portion of cephalon in front of glabella
Proparian:	facial suture with posterior branch cutting lateral margin of cephalon anterior of genal angle
Pygidium:	tail (posterior) portion of the body
Rostral plate:	middle portion of the ventral cephalic doublure, typically where the hypostome attaches
Schizochroal:	eyes with numerous compound lenses, each with individual cornea and separated by scleral walls. Common in phacopids	Schizochroal eye
Thoracic segment:	individual section of the thorax divided into axial ring and pleura
Thorax:	segmented body portion between cephalon and pygidium