calculus vs calculous

calculus is a noun but calculous is not a noun.

Word	Noun	Adjective	Verb	Adverb
calculus	Yes	No	No	No
calculous	No	Yes	No	No

calculus (noun)	calculous (noun)
the branch of mathematics that is concerned with limits and with the differentiation and integration of functions
a hard lump produced by the concretion of mineral salts; found in hollow organs or ducts of the body
an incrustation that forms on the teeth and gums

calculus (adjective)	calculous (adjective)
	relating to or caused by or having a calculus or calculi