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 during the Permian were of this order (Brachymetopidae and 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 [see Additional Notes below regarding elevation of Aulacopleurida as an order.]

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) and part of suggested basis for splitting this order into two (Aulacopleurida & Proetida). 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).
Other: anaprotaspides of Aulacopleuroidea and Bathyuroidea distinct from those of Proetoidea (Lerosey-Aubril, R., & R. Feist (2005) and part of suggested basis for splitting this order into two (Aulacopleurida & Proetida). See "Additional Notes" below.
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.

ADDITIONAL NOTES ON THE CLASSIFICATION OF PROETIDA:
The concept of the Order Proetida, which includes all of the stratigraphically youngest trilobites, is based on an ontogenetic character: protaspides with "adult-like" glabellae (e.g., elliptical or tapering in form, with a discernable preglabellar field)(Fortey & Owens 1975). The order is derived from the lower Ordovician Hystricurinae, once considered a ptychoparioid subfamily of mostly Laurentian species. Today, the family Hystricuridae is recognized by most workers, as closely related to the Dimeropygidae (in the Bathyuroidea). The family Proetidae can not be readily derived from that ancestor, suggesting another sister group among the late Cambrian ptychoparioids may be responsible. The unity of the Proetida depends on the "adult-like" glabella pattern of development evolving only once (Fortey 2001). The Proetoidea are distinctive and not easily linked to the other Proetida superfamilies, leading to proposals to split out that superfamily as nominate Order Proetida, and recognition of the Order Aulacopleurida (Adrain 2011) which he proposed would consist of the families Aulacopleuridae, Brachymetopidae, Dimeropygidae, Rorringtoniidae, Scharyiidae, Bathyuridae, Telephinidae, Holotrachelidae, and Hystricuridae, as well as the ptychopariid families Alokistocaridae, Crepicephalidae, Ehmaniellidae, Marjumiidae, Solenopleuridae, and Tricrepicehalidae.

However, in Lamsden & Selden (2015), a phylogenetic analysis did not support the establishment of Aulacopleurida, and strongly supported the Order Proetida as described here, united by the initial compound eye formation in early protaspides at the lateral margin (vs anterior), tapering protaspid glabella with a preglabellar field; adult characters such as quadrate or shield-shaped hypostome with regular posterior margin, hypostome median body entirely divided by a deep transverse groove, an enlarged 6th thoracic spine, 7-10 thoracic segments. Hystricurids and dimeropygids resolve at the base of the group, which divides into large proetoid and aulacopleuroid clades. Rorringtoniids and schyariids resolve as basal proetoids, rather than aulacopleuroids. They also criticize Adrain's minimal establishment of Order Aulacopleurida. Their paper provides a good review of the basis of the defining characters of Proetida that includes aulacopleuroids, but also recognize two large distinct  clades distinguishing proetoids and aulacopleuroids.

Lerosey-Aubril and Feist (2005) advanced the following: The status of the order Proetida (Fortey and Owens 1975) was questioned by Bergström (1977) who pointed out that many characters used for the proposed order Proetida were plesiomorphic. Consequently Fortey (1990) reduced the number of characters diagnostic of this order. More recently some doubts have emerged concerning the relationships of the different superfamilies of the Proetida. Although close phyletic relationships between the Aulacopleuroidea and Bathyuroidea are supported, the superfamily Proetoidea is more problematic.
Evidence from new silicified materials emphasises that the early ontogeny of Proetoidea is unique among the Proetida. This particular ontogeny is characterized by bubble-shaped and almost smooth anaprotaspides bearing three pairs of marginal spines, associated with a particularly broad ovoid hypostome provided with at least one prominent postero-medial spine, and followed by a maximum of two almost smooth adult-like metaprotaspid stages. Following several authors (Chatterton et al., 1999; Fortey 2001; Adrain, pers. com., 2003), it was recently proposed that proetoid trilobites should be separated from the other superfamilies belonging to the order Proetida (Lerosey-Aubril and Feist 2005). Proetoid trilobites appear to possess a unique early ontogeny that is conservative through time; all proetoid protaspides known from Ordovician to Early Carboniferous times display the same characteristic features as mentioned above.

Globular anaprotaspides occur in the ontogenies of a wide range of trilobite taxa but as stated by Chatterton et al. (1999) “they must be derived for the Proetida”. Since Speyer and Chatterton (1989), they have been interpreted as planktonic larvae and are considered to be separate from the subsequent benthic adult-like forms by a metamorphosis. Likewise a broad larval hypostome is a common ontogenetic feature in many trilobites but the anaprotaspid hypostomes of proetoid trilobites are remarkably large so that they almost fully cover the ventral opening of the larva without the prominent spines displayed in larval hypostomes of other taxa (Lerosey-Aubril and Feist 2005). Moreover, the proetoid larval hypostome displays a peculiar morphology characterized by an ovoid outline and a prominent postero-medial spine that extends beyond the doublure in in situ hypostomes (Lerosey-Aubril and Feist 2005). Finally proetoid metaprotaspides possess more heterogeneous morphologies but these are all very much “adult like” (sensu Fortey, 2001). More important is the fact that proetoid metaprotaspides are devoid of complex patterns of tuberculation as seen in many aulacopleuroid taxa and they never represent more than two stages during ontogeny. Each of these aspects should be carefully considered in a re-examination of the classification of the Proetida.

The lumping of the family Phillipsiidae into the Proetidae by Jell & Adrain 2003 has not been generally accepted by workers on the Proetida (e.g., Lerosey-Aubril and Feist 2005 continue to recognize Phillipsiidae). If Phillipsiidae is retained, the generic listing offered above for the Proetoidea should be split out into Proetidae, Phillipsiidae, and Tropidocoryphidae. The constituent genera of the retained Phillipsiidae would include advanced proetoids largely from the Carboniferous and Permian, but might include early members of that clade from the late Devonian. Otherwise, the species can be retained at the subfamilial level, in Phillipsiinae.