Missing plurals

-UM but not -A

Mass nouns: ALLUVIUM, AVIZANDUM, BDELLIUM, BUNKUM (also not Latin: named after Buncombe County in North Carolina), COLLUVIUM, COLOSTRUM, CORIUM, CORUNDUM, DELIRIUM, DENTALIUM, DEUTERIUM, DIDYMIUM, DOMINIUM, ELECTRUM, EPICARDIUM, EPIMYSIUM, FALCIPARUM, and most chemical elements (see below).

Chemical elements: ACTINIUM, ALUMINIUM, AMERICIUM, BARIUM, BERKELIUM, BERYLLIUM, BOHRIUM, CADMIUM, CAESIUM, CALCIUM, CERIUM, CHROMIUM, CURIUM, DUBNIUM, DYSPROSIUM, ERBIUM, EUROPIUM, FERMIUM, FRANCIUM, GADOLINIUM, GERMANIUM, HAFNIUM, HASSIUM, HELIUM, HOLMIUM, INDIUM, IRIDIUM, LAWRENCIUM, LUTETIUM, MEITNERIUM, MOLYBDENUM, NEODYMIUM, NEPTUNIUM, NIOBIUM, NOBELIUM, OSMIUM, PLATINUM, PLUTONIUM, POLONIUM, PROMETHIUM, RADIUM, RHENIUM, RHODIUM, RUBIDIUM, RUTHENIUM, SAMARIUM, SCANDIUM, SEABORGIUM, SELENIUM, SODIUM, TANTALUM, TECHNETIUM, TELLURIUM, TERBIUM, THALLIUM, THULIUM, TITANIUM, URANIUM, VANADIUM, YTTERBIUM, YTTRIUM. (Note that some chemical elements do admit plurals, but that's for another article.) Also COLUMBIUM (old term for NIOBIUM).

Adjectives: ANTEBELLUM, ANTEPARTUM, DINKUM (also adverb).

Plants (plants often do not admit Latinate plurals even when the root word is Latin): ALLIUM, ALYSSUM, ANTHURIUM, ARUM, CALADIUM, CAPSICUM, CERASTIUM, COLCHICUM, CYMBIDIUM, DELPHINIUM, DORONICUM, EPIMEDIUM, EPIPHYLLUM, ERYNGIUM.

Not Latin: BEGUM (from Urdu), BLUEGUM (compound with GUM), BOOJUM (nonsense word from The Hunting of the Snark), BUBBLEGUM, CARDAMUM (from Old French cardamome), CHECKSUM, CHETRUM (from Dzongkha), CHILLUM (from Hindi), COCKALORUM (original coinage from cock), DIAZONIUM (from German), DRUM (from Middle Dutch, Low German or Scottish Gaelic and Irish), DUMDUM (name of a town in India), EARDRUM, EUPHONIUM (from Greek, given Latinate ending).

Other:-

• ASYLUM (of Latin origin but for some reason plural is still ASYLUMS)
• COLISEUM and COLOSSEUM (alteration from Medieval Latin, plural is COLISEUMS/COLOSSEUMS)
• CONUNDRUM (unknown origin)
• CUM (preposition)
• DECORUM (noun in English but adjective in Latin. Plural is DECORUMS)
• DEDENDUM (noun in English but gerundive in Latin. Plural is DEDENDUMS)
• FACTOTUM (from Latin fac totum, "do the whole thing!". Plural is FACTOTUMS)
• FRAENULUM, FRAENUM, FRENULUM, FRENUM (Modern Latin, "little bridge". Plurals are FRAENULUMS etc.)
• FREEMIUM (original coinage from FREE + PREMIUM)

To do

ALARUM (mistake?), ALUM (mistake?), CEMENTUM

GALBANUM GALLIUM GELSEMIUM GERANIUM GEUM GLUM GUAIACUM GUM GYPSUM HABENDUM HAHNIUM HARMONIUM HELENIUM HOKUM HOODLUM HUMDRUM HYPERICUM IMPERIUM INDECORUM INFIMUM JEJUNUM JOLIOTIUM JORUM KETTLEDRUM KHOUM KUMKUM LABARUM LABDANUM LABURNUM LADANUM LANTHANUM LAUDANUM LIGUSTRUM LINOLEUM LOCUM LOMENTUM LUTECIUM LYCOPODIUM MAGNUM MECONIUM MEERSCHAUM MENTUM MODICUM MUSEUM MYOCARDIUM MYOMETRIUM NASTURTIUM NATATORIUM NEUM NGULTRUM NOSTRUM OAKUM ODIUM OLEUM OLIBANUM OPIUM OPOSSUM OPPROBRIUM ORICHALCUM ORIGANUM PABLUM PABULUM PANJANDRUM PANTOUM PASPALUM PENDULUM PEPLUM PERIMYSIUM PETROLATUM PETROLEUM PHORMIUM PLENUM PLUM POLYGONUM POMATUM POPADUM POPPADUM POSSUM POTASSIUM PRACTICUM PRAESIDIUM PRECORDIUM PREMIUM PRESIDIUM PROTIUM PSALTERIUM PSYLLIUM PUERPERIUM PYRETHRUM QUADRIVIUM QUORUM RHIZOBIUM RUM RUMDUM SANCTUM SARGASSUM SCHTUM SCRUM SCUM SEBUM SEDUM SESAMUM SHTUM SKOOKUM SLUM SOLANUM SORGHUM SPHAGNUM SPLENIUM SPUTUM STELLIUM STEPMUM STEWBUM STICKUM STRAMONIUM STRUM STUM STUMBLEBUM SUGARPLUM SUM SUPREMUM SWUM TALCUM TAPETUM TECTUM TEDIUM TEETOTUM THRUM TRIDUUM TRILLIUM TRITIUM TROPAEOLUM UROPYGIUM VALLUM VARIORUM VELLUM VERATRUM VERBASCUM VIBURNUM WAMPUM YESSUM.