os longum vs leg bone

os longum and leg bone both are nouns.

Word	Noun	Adjective	Verb	Adverb
os longum	Yes	No	No	No
leg bone	Yes	No	No	No

As nouns, leg bone is a hyponym of os longum; that is, leg bone is a word with a more specific, narrower meaning than os longum:

os longum: in limbs of vertebrate animals: a long cylindrical bone that contains marrow
leg bone: a bone of the leg

Other hyponyms of os longum include epiphysis, diaphysis, shaft, arm bone.

os longum (noun)	leg bone (noun)
in limbs of vertebrate animals: a long cylindrical bone that contains marrow	a bone of the leg