faeces vs melena

faeces and melena both are nouns.

Word	Noun	Adjective	Verb	Adverb
faeces	Yes	No	No	No
melena	Yes	No	No	No

As nouns, melena is a hyponym of faeces; that is, melena is a word with a more specific, narrower meaning than faeces:

faeces: solid excretory product evacuated from the bowels
melena: abnormally dark tarry feces containing blood (usually from gastrointestinal bleeding)

Other hyponyms of faeces include dog do, dog shit, dog turd, doggy do, crap, dirt, poop, shit, shite, turd, droppings, dung, muck, meconium, melaena.

faeces (noun)	melena (noun)
solid excretory product evacuated from the bowels	abnormally dark tarry feces containing blood (usually from gastrointestinal bleeding)