class Placodermi vs placoderm

class Placodermi and placoderm both are nouns.

Word	Noun	Adjective	Verb	Adverb
class Placodermi	Yes	No	No	No
placoderm	Yes	No	No	No

class Placodermi (noun)	placoderm (noun)
extinct group of bony-plated fishes with primitive jaws	fish-like vertebrate with bony plates on head and upper body; dominant in seas and rivers during the Devonian; considered the earliest vertebrate with jaws