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Irregular plurality situations in English

'sheep, deer, fish' are some of a row of animals where singular and plural have the same form, i.e. the plural (normally) has no -s. But of course there are cases when the normal plural with -s can be used.

Is there any information why there is such a peculiarity?

marked as duplicate by RegDwigнt Jul 8 '12 at 16:56

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