similarity vs dissimilar

similarity is a noun but dissimilar is not a noun.

Word	Noun	Adjective	Verb	Adverb
similarity	Yes	No	No	No
dissimilar	No	Yes	No	No

similarity (noun)	dissimilar (noun)
the quality of being similar
a Gestalt principle of organization holding that (other things being equal) parts of a stimulus field that are similar to each other tend to be perceived as belonging together as a unit

similarity (adjective)	dissimilar (adjective)
	not similar
	not alike or similar