Reconstruction of Wiwaxia corrugata, copyright James St. John.

Belongs within: Eumetazoa.
Contains: Coleolidae, Sachitida, Stenothecoida, Monoplacophora, Gastropoda, Bivalvia, Helcionellida, Rostroconchia, Cephalopoda.

The Mollusca include such animals as clams, snails, squids and slugs. Representatives have the body divided between a head-foot and visceral mass. Most have an open circulatory system. The foot is an elongate organ used for crawling locomotion and/or substrate attachment in basal forms. The visceral mass is overlain by a mantle. Molluscs often have a dorsal shell secreted by the mantle though this feature has been lost in numerous daughter lineages. The shell is primarily composed of aragonite with an outermost layer of protein (the periostracum). Molluscs generally have a ventral muscular foot for locomotion and a tooth-bearing radula for feeding, though again both characters may be lost or modified in various taxa. Ctenidia (gills) are often present in a cavity between the mantle and visceral mass.

Basal relationships between the various mollusc lineages have been subject to much debate, with a wide range of different arrangements suggested. One leading suggestion is a basal division between the Conchifera, which ancestrally possess a single shell (lost in some derived lineages, and divided into two valves in Bivalvia), and the Aculifera, which possess a body covering of spicules and may or may not have a shell composed of a series of separate valves. Alternatively, other authors have argued that the Aculifera are paraphyletic to the Conchifera. Relationships are, if anything, even more contentious within the Conchifera: almost every possible arrangement of the major subgroups has been proposed at one time or another. The name Pleistomollusca has been proposed for a clade supported by molecular data uniting the major classes Gastropoda and Bivalvia, but some recent analyses would also include the Scaphopoda (tusk shells) within this clade.

There are also a number of mostly Palaeozoic fossil groups of molluscs whose affinities remain obscure; many of these with simple, more or less cap-shaped shells have been classified in the past within a para- or polyphyletic taxon Monoplacophora. The early Cambrian Wiwaxia corrugata had a slug-like body bearing a dorsal covering of carbonaceous scales and elongate spines.

The Helcionelliformes are a group of univalved mollusks known from the early Cambrian. Their affinities have been subject to debate: some authors interpret them as the earliest members of the Gastropoda whereas others regard them as perhaps more basal among the mollusks and/or related to other living molluscan clades. Parkhaev (2002) regarded the turbospiral Pelagielliformes of the early to middle Cambrian as derived from helcionelliform ancestors.

The Helcionelliformes proper can be divided between the Yochelcionelloidea, in which the shell has the parietal train modified into a siphonal groove or tube, and the Helcionelloidea which lack such a feature (Parkhaev 2002).

Systematics of Mollusca
<==Mollusca (see below for synonymy)
| | |--Odontogriphus omalus Conway Morris 1976VP17, RE01
| | `--Bowengriphus Ritchie & Edgecombe 2001RE01
| | `--*B. perphlegis Ritchie & Edgecombe 2001RE01
| `--Wiwaxiidae [Wiwaxidae]H62
| |--Wiwaxia Walcott 1911VP17, Po08
| | `--*W. corrugata (Matthew 1899) [=Orthotheca corrugata]H62
| `--Pollingeria Walcott 1911H62
| `--*P. grandis Walcott 1911H62
`--Conchifera (see below for synonymy)VP17
| i. s.: CanopoconusEL11
| StenothecoidaR96
| Aeolidiopsis Pruvot-Fol 1956 [Aeolidiopsidae]BR17
| Archaeotremaria Yu 1979 (n. d.) [Archaeotremariidae, Archaeotremarioidea]BR17
| `--*A. polytremata Yu 1979 (n. d.)BR17
| Vitrella Clessin 1877BR17
| Crossopoma Martens 1891BR17
| `--*C. planorbulum (Lamarck 1816) (nom. cons) (see below for synonymy)BR17
| ‘Electra’ Albers 1850 non Lamouroux 1816BR17
| `--*E. ceylanica (Pfeiffer 1845) [=Achatina ceylanica]BR17
| Granoconus Yu 1979 (n. d.) [Granoconidae]BR17
| `--*G. trematus Yu 1979 (n. d.)BR17
| *Xerosecta explanata (Müller 1774)BR17 [=Helix explanataBR17; incl. Carocolla albellaG40]
| Peruinia Polinski 1922 [Peruiniinae, Peruiniini]BR17
| `--*P. peruana (Troschel 1847) [=Clausilia peruana]BR17
| Eucycloidea pulchella (d’Orbigny 1850) [=Purpurina pulchella]BR17
| Thoralispira laevisW01
| Kepinospira Yü 1963RJ76
| Pseudotheca Redlich 1899RJ76
| Siluracmaea Horny 1963RJ76
| Sinkiangodiscus Yü 1963RJ76
| Sinobellerophon Yü 1962RJ76
| Sinosinuites Yü 1963RJ76
| Decipula ovata Jeffreys in Friele 1875 [incl. Tellemya ovalis Sars 1878]N79
| AxinusN79
| |--A. croulinensis [incl. A. pusillus Sars 1868 (n. n.)]N79
| |--A. eumyarius Sars 1870N79
| `--A. ferruginosusN79
| ArtemisN79
| |--A. gigantea [=Cytherea (Artemis) gigantea; incl. A. ponderosa]C64
| `--A. linctaN79
| SyndosmyaN79
| |--S. albaD56
| |--S. nitidaN79
| |--S. ovataP75
| `--S. prismaticaN79
`--+--Pleistomollusca [Archaeobranchia, Cyclomya, Helcionelliformes, Helcionelloida, Homomyaria]KC11
| |--GastropodaTRM20
| `--Loboconcha [Diasoma]BRW98
| |--BivalviaTRM20
| `--+--HelcionellidaVP17
| `--RostroconchiaTRM20

Mollusca incertae sedis:
Succinodon (n. d.)N85
Harmandia Rochebrune 1882C92
Deltoideum deltaTP02
Bilobissa dilatataTP02
Umbella d’Orbigny 1841E03
Lapparia hebesA26
Gilbertia tertiariaA26
‘Dimorpha’ Gray 1840 non Panzer 1806C-S93
Janella Grateloup 1838BR05
Lampania zonalisF66
Gladius fususF66
Macha strigillataF66
Trigonella luzonicaF66
Cryptogramma flexuosaF66
Tropicolpus albolapisM49
Zeacuminia pareoraensisM49
Bartrumia tenuiplicataM49
Zippora membranaceaP75
‘Eatonia’ Smith 1875 non Hall 1857S61
Festilyria festiva (Lamarck 1811)ES88
Somalipecten Waller 1986ES88
`--*S. cranmerorum Waller 1986ES88
‘Margarita’ Lea 1836 nec Leach 1814 nec Leach 1819LT64
‘Margarita’ Lea 1838 nec Leach 1914 nec Leach 1819 nec Lea 1836LT64
Cemoria princeps Mighels & Adams 1842J49
Badiotella Bittner 1890M65
Curvulites Rafinesque 1831M65
‘Macquartia’ Roullier & Vorinsky 1848 non Robineau-Desvoidy 1830M65
Physodon integrumJB78
Triton Linnaeus 1758L58
|--*T. littoreus Linnaeus 1758L58
|--T. costatusB89
|--T. lignariusC64
|--T. nodiferPP64
|--T. pilearisC64
|--T. scaberC64
`--T. vestitusC64
Medusa Linnaeus 1758L58
|--M. aequorea Linnaeus 1758L58
|--M. brachiata Linnaeus 1758L58
|--M. cruciata Linnaeus 1758L58
|--M. marsupialis Linnaeus 1758L58
|--M. pelagica Linnaeus 1758L58
`--M. pilearis Linnaeus 1758L58
Eleuchus vulgaris Adams 1853 (n. d.)H08
Irondalia Stinchcomb & Angeli 2002TS11
Bestashella Missarzhevsky in Missarzhevsky & Mambetov 1981TS11
Kistasella Missarzhevsky 1989TS11
Tianzhushanellidae [Siphonoconcha]WS93
|--Tianzhushanella ovata Liu 1979WS93
|--A. aphelesPa08
|--A. presiphonalisPa08
`--A. siphonalis Conway Morris & Bengtson 1990WS93
Aroonia sepositaPa08
Drillus maravignaePP64
Myella bidentataPP64
|--S. terebralis (Gould 1860) [=Neptunea (Sipho) terebralis]C64
`--S. torusPP64
Pleurobranchon meckeliPP64
Rupertia Gray 1865LT61
Williamsia Carus 1890B61
Lysechinus Gregory 1896SG93, FP66
`--*L. incongruens Gregory 1896FP66
Bukobia cockerelliPB27
Tryonia clathrataPB27
Carychiopsis schwageriE99
Canalicia attractaE99
Deperetia Teppner 1921G00
Pecchiolia abyssicolaN79
Acteonia corrugataN79
Colobocephalus costellatusN79
|--C. maeoticaPP64
`--C. steenstrupiiN79
Magdala arcticaN79
|--P. badgerensisP79a
`--P. striatulaP79a
|--T. dubia Tennison Woods 1877P79a
|--T. mariae Tennison Woods 1877P79a
`--T. tesselata Tennison Woods 1877P79a
Strophia uvaG79a
|--C. calcareusG79b
|--C. kraussianusG79b
`--C. zanguebaricusG79b
Diplomphalus megeiP79b
|--M. decisa [incl. M. decisa var. melanostoma Currier in Walker 1879 (n. n.)]W79
|--M. integraW79
|--M. milesiiW79
|--M. obesaW79
|--M. ponderosaW79
`--M. rufaW79
Stundella nasutaC64
|--B. mutabilisC64
`--B. pacificaC64
Carinea emarginataC64
Alora gouldii Ad. & Ad. 1856 [=Trichotropis gouldii]C64
Levenia coarctataC64
Rhizocheilus nux [incl. R. californicus, R. distans, Purpura osculans]C64
Clavella distortaC64
Margaritiphora fimbriata [incl. Avicula barbata, A. cumingii, Margaritiphora mazatlanica]C64
Argina brevifronsC64
Aricula libellaC64
Mumiola ovataC64
Poronia rubraC64
Strephona pedroana Conrad 1855C64
Aeneta harpaC64
Orthygia Mörch 1853RC02
Phoenicobius Moerch 1852V02
Vexillitra marwickiB70

Nomen nudum: Cemoria candida Mighels & Adams 1841J49

Conchifera [Ancyropoda, Cephalaea, Cyrtonellea, Cyrtonellina, Cyrtosoma, Ganglioneura, Perocepala, Rhacopoda, Tubicolae, Uberes, Visceroconcha]VP17

*Crossopoma planorbulum (Lamarck 1816) (nom. cons) [=Cyclostoma planorbulum, Cyclotus planorbulus, Cyclotus planorbutus (l. c.); incl. Crossopoma cornuvenatorium (Gmelin 1791)]BR17

Mollusca [Adenopoda, Amphineura, Archinacellida, Archinacellina, Branchifera, Coponautae, Cyclobranchiata, Cynostraca, Delocephala, Dermobranchea, Dipteronautae, Eumollusca, Exocephala, Glossophora, Halwaxiida, Hepagastralia, Isopleura, Monoconcha, Orthodonta, Phanerogama, Prosopocephala, Scutibranchiata, Serialia, Solenoconchia, Spiculata, Testaria, Univalvia]

*Type species of generic name indicated


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