ventral fin(n = noun.animal) pelvic fin - either of a pair of fins attached to the pelvic girdle in fishes that help control the direction of movement; correspond to hind limbs of a land vertebrate;
is a kind of fin

caudal fin, dorsal fin, fin, fin de si'8acle, fin de siecle, fin keel, fin whale, heterocercal fin, homocercal fin, pectoral fin, pelvic fin, tail fin, ventral, ventral placentation, vertical fin