holocephalian vs chondrichthian

holocephalian and chondrichthian both are nouns.

Word	Noun	Adjective	Verb	Adverb
holocephalian	Yes	No	No	No
chondrichthian	Yes	No	No	No

As nouns, chondrichthian is a hypernym of holocephalian; that is, chondrichthian is a word with a broader meaning than holocephalian:

holocephalian: fish with high compressed head and a body tapering off into a long tail
chondrichthian: fishes in which the skeleton may be calcified but not ossified

Other hypernyms of holocephalian include cartilaginous fish.

holocephalian (noun)	chondrichthian (noun)
fish with high compressed head and a body tapering off into a long tail	fishes in which the skeleton may be calcified but not ossified