Conchologists mainly deal with four molluscan orders : the gastropods ( snails ), bivalves ( clams ), Polyplacophora ( chitons ) and Scaphopoda ( tusk shells ).
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Furthermore, there are 60 crayfish, 699 types of soft animals ( species of mollusks ) consisting of 530 snails ( gastropoda ), 159 shells ( bivalve ), 2 scaphopoda, 5 squid ( cephalopoda ) and 3 chiton.
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However, an analysis in 2009 using both morphological and molecular phylogenetics comparisons concluded the molluscs are not monophyletic; in particular, Scaphopoda and Bivalvia are both separate, monophyletic lineages unrelated to the remaining molluscan classes; the traditional phylum Mollusca is polyphyletic, and it can only be made monophyletic if scaphopods and bivalves are excluded.
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Most members of the scaphopoda have shells that exhibit a noticeable and characteristic anterior curvature the shell of " R . rectius ", however, is unusually straight, hence the Latin word " rectius " ( literally, " straight " ) in its binomial designation . " R . rectius " is a generalist carnivore found in shallow silty and sandy substrates.
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