Introduction: typically small trilobites, separated from Ptychopariida in Fortey & Owens 1975; one of Fortey's (1990) "Libriostomate" orders (with natant hypostome or derived therefrom); exoskeleton sometimes with pits or small tubercles. The last of the trilobites was of this order (Phillipsiinae).
Cephalon: opisthoparian sutures; glabella large, vaulted, well-defined, typically narrowing forwards, typically 4 pairs of glabellar furrows with posterior-most pair longest and deepest, those anterior shorter and fainter; eyes, usually present, holochroal, often large, convex; rostral plate narrow and backward tapering; long hypostome, most species natant, but some secondarily conterminant (e.g., late Proetidae & Bathyuridae); typically with genal spines.
Thorax: 7 – 22 (typically 10) segments, tips variable, blunt to long-spined.
Pygidium: micropygous to subisopygous, often spineless, and usually with 4 – 10+ distinct pleural furrows. 
Other: metaprotaspides show early development of "adult-like" (elliptical/tapering) glabella and preglabellar field (see example left)
Occurrence: Ordovician (Tremadoc) to Permian (Tartarian)
Suborders: None (or nominate Proetina)
Superfamilies: Proetoidea, Aulacopleuroidea, Bathyuroidea 

Cephalon: opisthoparian sutures, glabella tapering or inverse pyriform, mostly suboval, with 3-4 pairs of lateral furrows, sometimes indistinct (e.g., Proetinae), or glabella long, expanding forward to anterior border furrow or beyond, with lateral preoccipital lobes present (e.g., Phillipsiinae); eyes, when present, typically holochroal, convex; fixigenae narrow, librigenae broad (except in blind species); hypostoma typically natant, but secondarily conterminent in advanced Proetidae; genal angle spined or blunt rounded.
Thorax: 7-10 segments, typically 10 in Proetinae, 9 in Phillipsiinae, pleurae with furrows, tips blunt or spined.
Pygidium: from small and hemispherical, with few segments (e.g., Proetinae), to long, parabolic, with up to 33 axial segments (e.g., Phillipsiinae); margin typically smooth, but some with terminal axial spine or pleural spines. 
Other: anaprotaspides of Proetoidea distinct from those of Aulacopleuroidea and Bathyuroidea (Lerosey-Aubril, R., & R. Feist (2005). Difficulties in finding consistent characters diagnostic of adult Proetoidea are discussed in greater length in "Additional Notes" below.
Families: Proetidae (subfamilies Proetinae & Phillipsiinae -- alternately Phillipsiidae treated as separate family, see "Additional Notes" below), Tropidocoryphidae
Genera: Proetidae: Aayernenaytcheia, Acanthophillipsia, Aceroproetus, Acropyge, Alaskalethe, Altajaspis, Ameropiltonia, Ameura, Ampulliglabella, Anambon, Anglibole, Angustibole, Anisopyge, Anujaspis, Appendicysta, Aprathia, Archaeocoryphe, Archegonus (=Cylindraspis), Ascetopeltis, Astroproetus (=Clypoproetus; =Enodiproetus; =Sibiroproetus), Australokaskia, Bailielloides, Bapingaspis, Basidechenella, Bedicella, Beleckella, Belgibole, Benesovella, Bitumulina, Blodgettia, Bohemiproetus, Bolivicrania, Boliviproetus, Bollandia, Bonnaspidella, Borealia, Brevibole, Breviphillipsia, Burgesina, Calybole, Camsellia, Capricornia, Carbonocoryphe, Carbonoproetus, Carlopsia, Carniphillipsia, Ceratoproetus, Chauffouraspis, Chaunoproetoides, Chaunoproetus (=Carnicia), Chiides, Chiops, Chlupacula, Chuanqianoproetus, Clavibole, Combewoodia, Comptonaspis, Coniproetus, Conophillipsia, Constantina, Craspedops, Crassibole, Crassiproetus, Cummingella, Cyphinioides, Cyphoproetus, Cyrtodechenella, Cyrtoproetus, Cyrtosymbole, Cystispina, Daihuaia, Dayinaspis, Dechenella (/Eudechenella), Dechenelloides, Dechenellurus, Deinoproetus, Delaria, Deltadechenella, Diabole, Diacoryphe, Ditomopyge (=Cyphinium; =Permoproetus; =Neophillipsia), Doublatia, Drevermannia, Dudu, Dushania, Effops, Ejinoproetus, Elegenodechenella, Elimaproetus, Elliptophillipsia, Endops, Ensecoryphe, Eocyphinium, Eocyrtosymbole, Eodrevermannia, Eomicrophillipsia, Eopalpebralia, Eosoproetus, Eowinterbergia, Erbenaspis, Erbenites, Evagena, Exochops, Flexidechenella, Formonia, Francenaspis, Franconicabole, Frithjofia, Fuscinipyge, Ganinella, Gapeevella, Geigibole, Georhithronella, Gerastos (=Dohmiella; =Kegeliella), Gitarra, Globusia, Globusiella, Globusoidea, Gomiites, Gracemerea, Griffithidella, Griffithides, Grossoproetus, Haasia, Hamiroproetus, Hassiabole, Hedstroemia (=Milesdavis; =Pachyproetus), Helioproetus, Helmutia, Helokybe, Hentigia, Hesslerides, Hildaphillipsia, Humeia, Humilogriffithides, Hunanoproetus, Hypaproetus, Iranaspidion, Jimbokranion, Jinia, Karginella, Kaskia, Kathwaia, Kerpenella, Khalfinella, Kollarcephalus, Kolymoproetus, Kosovoproetus, Krambedrysia, Kulmiella, Kulmogriffithides, Lacunoporaspis, Laevibole, Langgonbole, Latibole, Latiglobusia, Latiproetus, Lauchellum, Lichanocoryphe, Linguaphillipsia, Liobole, Liobolina, Longilobus, Longiproetus, Lophiokephalion, Lugalella, Luojiashania, Macrobole, Mahaiella, Malayaproetus, Malchi, Mannopyge, Megaproetus, Menorcaspis, Merebolina, Metaphillipsia, Microphillipsia, Microspatulina, Mirabole, Monodechenella, Moravocoryphe, Moschoglossis, Myoproetus, Namuraspis, Neogriffithides (=Siciliproetus), Neokaskia, Neoproetus, Nipponaspis, Nitidocare, Nodiphillipsia, Novoameura, Nunnaspis, Oehlertaspis (/Oehlertia), Oidalaproetus, Orbitoproetus, Ormistonaspis, Omlistonia, Ormistoniella, Osmolskia, Otodechenella, Paladin (=Weberides), Palaeophillipsia, Paleodechenella, Palpebralia, Panibole (=Proliobole), Parachaunoproetus, Paradechenella, Parafrithjofia, Paraglobusia, Paragriffithides, Paramirabole, Parangustibole, Parapalpebralia, Paraphillipsia, Paraproetus, Parawarburgella, Particeps, Parvidumus, Paryfenus (/Colymbus), Pedinocoryphe, Pedinodechenella, Perexigupyge, Perliproetus, Phillibole, Phillibolina, Philliboloides (/Phillibolina), Phillipsia, Phyllaspis, Piltonia, Planilobus, Planokaskia, Plesiowensus, Podoliproetus, Pontipalpebralia, Praedechenella, Pragoproetus, Prantlia (=Malvernocare), Prodiacoryphe, Proetocephalus, Proetus (/Euproetus; /Aeonia; =Devonoproetus; =Falcatoproetus; =Forbesia; =Scotoproetus; =Trigonaspis), Pseudobollandia, Pseudocyrtosymbole, Pseudodechenella (=Arcticormistonia), Pseudodudu, Pseudogerastos, Pseudophillipsia, Pseudoproetus, Pseudosilesiops, Pseudospatulina, Pseudowaribole, Pudoproetus (=Zhifangia), Pulcherproetus, Pusillabole, Raerinproetus, Reediella, Rhenocynproetus, Rhenogriffides, Richterella, Rosehillia, Rugulites (/Podolites), Ryckholtia, Schaderthalaspis, Schizophillipsia, Schizoproetina, Schizoproetoides, Schizoproetus, Serniproetus, Sevillia, Silesiops, Simaproetus, Sinobole, Sinocyrtoproetus, Sinopaladin, Sinoproetus, Sinosymbole, Skemmatocare, Skemmatopyge, Spatulina, Spergenaspis, Spinibole, Spinibolops, Struveproetus, Sulcubole, Tawstockia, Taynaella, Tcherkesovia, Tetinia, Thaiaspella, Thaiaspis, Thalabaria, Thebanaspis, Thigriffides, Timoraspis, Triproetus, Tropiconiproetus, Tropidocare, Tschernyschewiella (/Schmidtia), Typhloproetus, Unguliproetus, Vandergrachtia, Vidria, Vittaella, Wagnerispina, Waideggula, Waigatchella, Warburgella (/Holometopus; =Owensella), Waribole, Weania, Weberiphillipsia (=Spinolimbella), Westropia, Weyeraspis, Winiskia, Winterbergia, Witryides, Xenadoche, Xenoboloides, Xenocybe, Xenodechenella, Xiangzhongella, Xiushuiproetus, Yanshanaspis, Yichangaspis, Yishanaspis, Zhegangula, Zhejiangoproetus.

Cephalon: semicircular or semielliptical, with border and (typically wide) preglabellar field present; glabella typically short; eyes variable, with or without defined eye ridges, genal spines present; hypostome natant (primitive condition); exoskeleton pitted, or with small tubercles.
Thorax: 11-22 segments, pleural ends usually rounded, may bear axial spine.
Pygidium: may be small, short, with even margin (Aulacopleuridae), or longer, and with spines (some Brachymetopidae).
Families: Aulacopleuridae, Brachymetopidae, Rorringtoniidae
Genera: Aulacopleuridae: Aulacopleura (/Arethusa/Arethusina; =Paraaulacopleura), Aulacopleuroides, Beggaspis, Coignops, Cyphaspides, Cyphaspis (=Novakaspis), Dixiphopyge, Harpidella (=Rhinotarion), Latecephalus, Malimanaspis (=Goodsiraspis), Maurotarion (=Goniopleura; =Branisella; =Tricornotarion), Namuropyge (=Coignouina), Otarion (=Aulacopleurella; =Conoparia; =Otarionella), Otarionides, Protocyphaspides, Pseudotrinodus, Songkania, Tilsleyia.

Cephalon: convex, with distinct border of varying width, opisthoparian sutures; glabella well defined, subparallel or gently expanding/tapering forward, with 3 or fewer lateral furrows, sometimes faint or absent; eyes medium to large, placed opposite or behind center of glabella; hypostoma typically natant, but secondarily conterminent in advanced Bathyuridae; genal spines typically present, sometimes broad.
Thorax: 8-12 segments, pleurae typically with distinct furrows.
Pygidium: subisopygous to micropygous, gently to moderately convex, no border furrow, axis of varying length (absent in Celmidae), sometimes prolonged into post-axial ridge or axial spine, often 3 axial rings + terminal; exoskeleton sometimes ornamented variously with pits, tubercles, small spines, or terraces.
Families: Bathyuridae, Dimeropygidae (including Celmidae), Holotrachelidae, Hystricuridae, Raymondinidae (including Glaphuridae), Telephinidae, Toernquistiidae.
Genera: Bathyuridae: Acidiphorus (=Goniotelina; =Goniotelus/Goniurus), Aksuaspis, Bathyurellus, Bathyurus, Benthamaspis (=Oculomagnus), Bolbocephalus, Catochia, Ceratopeltis, Eleutherocentrus, Ermanella, Gignopeltis, Grinnelaspis, (/Actinopeltis Poulsen), Hadrohybus, Jeffersonia (=Bathyurina), Licnocephala (=Domina), Lutesvillia, Madaraspis, Peltabellia (=Biolgina), Petigurus, Platyantyx, ?Proscharyia, Psephosthenaspis (=Aponileus; =Ludvigsenella), Pseudoolenoides, Punka, Rananasus, Randaynia, Raymondites, Sinobathyurus, Strigigenalis, Uromystrum.