selachian vs chondrichthian

selachian and chondrichthian both are nouns.

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

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

selachian: any of numerous fishes of the class Chondrichthyes characterized by a cartilaginous skeleton and placoid scales: sharks, rays, skates
chondrichthian: fishes in which the skeleton may be calcified but not ossified

Other hypernyms of selachian include cartilaginous fish.

selachian (noun)	chondrichthian (noun)
any of numerous fishes of the class Chondrichthyes characterized by a cartilaginous skeleton and placoid scales: sharks, rays, skates	fishes in which the skeleton may be calcified but not ossified