os longum vs arm bone

os longum and arm bone both are nouns.

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

As nouns, arm bone is a hyponym of os longum; that is, arm 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
arm bone: a bone in the arm

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

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