Simplified relationship diagram of mollusc's evolution: In 1832, while travelling on the Beagle, naturalist Charles Darwin collected giant fossils in South America. ...

Cladogram shows hypothetical mollusc's evolution. Greek clados = branch) or phylogenetic systematics is a branch of biology that determines the evolutionary relationships of living things based on derived similarities. ... Classes Caudofoveata Aplacophora Polyplacophora Monoplacophora Bivalvia Scaphopoda Gastropoda Cephalopoda † Rostroconchia † Helcionelloida The mollusks (American spelling) or molluscs (British spelling) are the large and diverse phylum Mollusca, which includes a variety of familiar animals well-known for their decorative shells or as seafood. ...

Vertical cladogram (modified from: ^[1]):

 Polyplacophora Gastropoda Rostroconchia Aplacophora Monoplacophora  Cephalopoda  Scaphopoda Bivalvia     /   /     /   /     /   /    /  /    * Cyrtosoma  /      /      /     /     * Diasoma     /     /     /     /     /     /    /    /    /    /   /   * Conchifera   /   /  /  * Testaria  /  / / / hypothetical ancestral mollusk 

Horizontal cladogram (source: ^[2]):

 Mollusca molluscs | +-- Amphineura | | | +-- Aplacophora | | | +-- Solenogastres | +-- Caudofoeveata | +-- Testaria | +-- Polyplacophora - chitons | | | +-- Palaeoloricata | +-- Neoloricata | +-- Conchifera | +-- Monoplacophora | +--+ | +--Cyrtosoma | | | +-- Gastropoda - gastropods | | | +-- Cephalopoda - cephalopods +-- Diasoma | +-- Rostroconchia | +--+ | +-- Scaphopoda - tusk shells | +-- Bivalvia - bivalves 

Subclasses and families Subclass Chaetodermomorpha (Caudofoveata) Family Chaetodermidae Family Falcidentidae Family Limifossoridae Family Metachaetodermatidae Family Prochaetodermatidae Family Scutopidae Subclass Neomeniomorpha (Solenogastres) Family Acanthomeniidae Family Amphimeniidae Family Dondersiidae Family Drepanomeniidae Family Epimeniidae Family Gymnomeniidae Family Hemimeniidae Family Heteroherpiidae Family Imeroherpiidae Family Lepidomeniidae Family Macellomeniidae Family Meiomeniidae Family Neomeniidae Family Perimeniidae Family... Lined Chiton (Tonicella lineata) Chitons, also called polyplacophorans and rarely polyplacophores, are 860 species of molluscs of the Class Polyplacophora. ... Orders Cyrtonellida 1987 Tryblidiida 1987 [Tryblidioidea] Tryblidiida 1982 [Bellerophontida] 1987 Pelagiellida 1987 Monoplacophora is a class of Mollusks thought to be extinct until 1952, when a living animal was dredged up from deep marine sediments. ... Subclasses Eogastropoda (True Limpets and relatives) Orthogastropoda The gastropods, gasteropods, or univalves, are the largest and most successful class of mollusks, with 60,000-75,000 known living species comprising the snails and slugs as well as a vast number of marine and freshwater species. ... Orders Sepiida Sepiolida Spirulida Teuthida Octopoda Vampyromorphida Nautilida The Cephalopods (head-foot) are the mollusc class Cephalopoda characterized by bilateral body symmetry, a prominent head, and a modification of the mollusc foot into the form of arms or tentacles. ... Orders See text The Rostroconchia is a class of extinct mollusks dating from the early Cambrian to the late Permian. ... Orders Dentaliida Gadilida The tusk shells are a class Scaphopoda of marine mollusks distinguished by curved tubular shells open at both ends, resembling a elephants tusk (thus the name). ... Orders Subclass Protobranchia Solemyoida Nuculoida Subclass Pteriomorphia - oysters, mussels Arcoida Mytiloida Pterioida Subclass Paleoheterodonta Trigoinoida Unionoida - freshwater mussels Subclass Heterodonta - clams, zebra mussels Veneroida Myoida Subclass Anomalosdesmata Pholadomyoida Bivalves are molluscs belonging to the class Bivalvia. ...

References

1. ^ EES 227: Paleobiology (Spring 2004) http://ethomas.web.wesleyan.edu/wescourses/2004s/ees227/01/227lect18.htm
2. ^ Zoology 250 Phylogenetic Trees (2006) http://www.biology.ualberta.ca/courses.hp/zool250/Clades/clade08-Mollusca.htm