The plural of genius, as applied to a man of unusual vigor of mind, is geniuses. When applied to a good or bad spirit, the plural is genii.
Formula
Formulas, larvas, stigmas, are regular English plurals; formulae, larvae, and stigmata are the classical plurals. Nebulae and alumnae are the proper plurals, the latter being the feminine noun corresponding to the masculine plural alumni.
Datum, Phenomenon
Datum, erratum, candelabrum, and memorandum form their plurals by changing um to a; as, data, errata, etc. The last two also take the English plurals, memorandums, candelabrums.
The plural of phenomenon and criterion are phenomena, criteria, although criterions is sometimes employed.
The plural forms, data, strata, and phenomena, are so much more frequently used than their singular forms, datum, stratum, and phenomenon, that some writers have slipped into the habit of using the plurals with a singular meaning; as, “The aurora borealis is a very strange phenomena.” “Our data is insufficient to establish a theory.” “The strata is broken and irregular.”
While most words ending in man become plural by changing this termination to men, as gentlemen, noblemen, clergymen, statesmen, the following simply add s: dragomans, Mussulmans, Ottomans, talismans “A dozen dragomans offered their services as guides and interpreters.” “A band of Mussulmans cut off our retreat.” “Those fierce Ottomans proved to be very revengeful.” “He purchased five finely upholstered ottomans for his drawing-room.”
Heroes, Cantos