Page 548 - Practical English Usage 3ed - Michael Swan, Oxford

Singular
barracks series crossroads species
Plural
barracks series crossroads species
Singular
headquarters works (= factory) means
Swiss
Plural
headquarters works
means
Swiss
singular and plural (2): irregular and special plurals 524
3 plural same as singular
Some words ending in -s do not change in the plural. Common examples:
Note that some singular uncountable nouns end in -so These have no plurals. Examples are news, billiards, draughts (and some other names of games ending in -s), measles (and some other illnesses).
Most words ending in -ics (e.g. mathematics, physics, athletics) are normally singular uncountable and have no plural use.
Too much mathematics is usually taught in schools. (NOT TBB '"tiny tntlthematies tlre ...)
Some words ending in -ics (e.g. politics, statistics) can also have plural uses. Politics is a complicated business. (B U T What are your politics?) Statistics is useful in language testing. (BUT The unemployment statistics
are disturbing.)
Other nouns which do not change in the plural are craft (meaning 'vehicle'), aircraft, hovercraft, spacecraft, Chinese, Japanese (and other nationality nouns ending in -ese), sheep, deer, fish, and the names of some other living creatures, especially those that are hunted or used for food. Fish has a rare plural fishes, but the normal plural is fish.
Dozen, hundred, thousand, million, stone (= 14 pounds) and foot (= 12 inches) have plurals without -s in some kinds of expressions. For details, see 389.15.
Dice (used in board games) is originally the plural of die, which is not now often used in this sense; in modem English dice is generally used as both singular and plural.
Data is originally the plural of datum, which is not now used. In modem English data can be used either as an uncountable noun (this data is ...) or as a plural (these data are .. .), with no difference of meaning.
Media is originally the plural of medium. The plural expression the media (meaning 'radio, 1V, newspapers, the internet .. .') is now quite often used as an uncountable noun with a singular verb.
4 foreign plurals
Some words which come from foreign languages have special plurals. Examples:
Singular
analysis appendix bacterium basis cactus corpus crisis
Plural
analyses (Latin)
appendices (Latin)
bacteria (Latin)
bases (Greek)
cacti (Latin) or cactuses (less common) corpora (Latin)
crises (Greek)
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