I'm trying to recall a story where humans have died off. Dolphins are repopulating the land and a series of surgical procedures is involved. The final procedure is to close the blowhole.
There may have been a "love story" where one dolphin was going on land and the other was staying behind.
I believe that this was in a collection of short stories (possibly including "Examination Day") from the mid seventies. The story would have been targeted at seventh and eight graders.