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The glossary provided here is largely based on “Seashells of Southern Florida: Bivalves,” by Paula M. Mikkelsen & Rüdiger Bieler, Princeton University Press. Production of this glossary was funded in part by the National Science Foundation’s Partnerships in Enhancing Expertise in Taxonomy [PEET] Program, NSF-DEB-9978119, “Bivalves - Research, Training, Electronic Dissemination of Data” (1999-2008) to Bieler and Mikkelsen.

M

MANTLE
MANTLE fig.Fleshy outer tissue surrounding the organs of a molluscan body and secreting the PERIOSTRACUM and shell; consisting of two lobes in a bivalve, one lining each shell, and at the ventral edge 2-4 folds that may have different functions or features (figure: Mercenaria mercenaria). See also SIPHONS for types of mantle fusion proposed by Yonge (1957).
MANTLE CAVITY
Chamber between the MANTLE lobes and interior visceral mass, containing CTENIDIA, etc.; also called pallial cavity.
MARGINAL DENTICLES
MARGINAL DENTICLES fig.Teeth or ridges on the free interior valve margins of a non-gaping bivalve shell that interlock when the valves are fully closed (figure: Astarte smithii).
MARSUPIUM (pl. MARSUPIA)
Specialized incubatory chamber for brooded larvae.
MESOPLAX
MESOPLAX fig.Accessory shell plate in Pholadidae and Teredinidae posterior to the PROTOPLAX, both protecting the anterior ADDUCTOR MUSCLE (figure: Cyrtopleura costata).
MESOSOMA
See METASOMA.
METAPLAX
METAPLAX fig.Accessory shell plate in Pholadidae and Teredinidae extending posteriorly between the two valves from the UMBONES (figure: Martesia striata).
METASOMA
METASOMA fig.Posteroventral sack-like extension of the visceral mass containing the STYLE SACK of the STOMACH and part of the gonad (in Ostreidae and Mytilidae); also called mesosoma, pyloric process (figure: Mytilidae).
MIDGUT
MIDGUT fig.Anterior portion of the intestine, here applied to that part within the visceral mass; in text, "coiled" indicates that one or more loops occur (figure: Corbiculidae).
MONOMYARIAN
MONOMYARIAN fig.Having a single ADDUCTOR MUSCLE or single muscle scar (in some cases formed by the coalesced insertions of several muscles) (figure: Gryphaeidae).
MONOPHYLETIC
(pertaining to a taxonomic group) defined by SYNAPOMORPHIES and including an ancestor and all of its descendants.
MOTHER-OF-PEARL
See NACRE.
MULTIVINCULAR
MULTIVINCULAR fig.Type of dorsal LIGAMENT that consists of serially repeated lamellar and fibrous elements (of the ALIVINCULAR type) across the HINGE line, typically OPISTHODETIC in position, and occurring in such families as Isognomonidae.
MUSCLE SCAR
MUSCLE SCAR fig.Impression on the shell interior that indicates the attachment position of a muscle (figure: Plicatula gibbosa).
MYOPHORE
Generalized term for shelly structure by which muscles attach to the shell, often in the form of a ridge or shelf; when enlarged, often called SEPTUM or MYOPHORIC RIDGE/flange/buttress; see also APOPHYSIS.
MYOPHORIC BUTTRESS
See MYOPHORE.
MYOPHORIC FLANGE
See MYOPHORE.
MYOPHORIC RIDGE
See MYOPHORE.
MYOSTRACUM
Irregularly PRISMATIC aragonite deposited at sites of muscle attachment to the shell.
MYTILIFORM
MYTILIFORM fig.Mussel-shaped (figure: Ischadium recurvum).