six-gilled shark vs cow shark

six-gilled shark and cow shark both are nouns.

Word	Noun	Adjective	Verb	Adverb
six-gilled shark	Yes	No	No	No
cow shark	Yes	No	No	No

As nouns, six-gilled shark and cow shark are synonyms defined as:

six-gilled shark and cow shark: large primitive shark widely distributed in warm seas

Other synonyms of six-gilled shark include Hexanchus griseus.

six-gilled shark (noun)	cow shark (noun)
large primitive shark widely distributed in warm seas	large primitive shark widely distributed in warm seas