In a few minutes they were close enough to note that the entire bottom of the ocean in the area where the creature had been seen had gone suddenly dark; and in the translucent depths above nearly all of the party discerned a gigantic shadow moving along.  It looked for all the world like an immense pancake with bat-like wings.  These wings were fluttering queerly, and from the action of the fish Mr. Choate said he was sure it was devouring prey which it had just killed.  He now asked Paul if he would like to try a cast.  The boy assented eagerly. Bracing his feet in the bottom of the motor-boat he took good aim and let his harpoon fly.