cetaceous vs cetacean

cetaceous is not a noun while cetacean is a noun.

Word	Noun	Adjective	Verb	Adverb
cetaceous	No	Yes	No	No
cetacean	Yes	Yes	No	No

As adjectives, cetaceous and cetacean are synonyms defined as:

cetaceous (noun)	cetacean (noun)
	large aquatic carnivorous mammal with fin-like forelimbs no hind limbs, including: whales, dolphins, porpoises, narwhals

cetaceous (adjective)	cetacean (adjective)
of or relating to whales and dolphins etc	of or relating to whales and dolphins etc