periwinkle vs myrtle

periwinkle and myrtle both are nouns.

Word	Noun	Adjective	Verb	Adverb
periwinkle	Yes	No	No	No
myrtle	Yes	No	No	No

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

periwinkle: chiefly trailing poisonous plants with blue flowers
myrtle: widely cultivated as a groundcover for its dark green shiny leaves and usually blue-violet flowers

Other hyponyms of periwinkle include Vinca minor, large periwinkle, Vinca major.

periwinkle (noun)	myrtle (noun)
edible marine gastropod	any evergreen shrub or tree of the genus Myrtus
small edible marine snail; steamed in wine or baked	widely cultivated as a groundcover for its dark green shiny leaves and usually blue-violet flowers
chiefly trailing poisonous plants with blue flowers
commonly cultivated Old World woody herb having large pinkish to red flowers