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Their external features are an elongated pisciform body, the toes joined by a membrane converting the feet into broad flippers or fins, the two hind ones being so close as to act like the caudal fin of a fish.

DESCRIPTION.—Body pisciform, terminated by a horizontal fin with two lobes; colour slaty brown above, sometimes bluish black, whitish below.

On the other hand the "non-rigid" type may be roughly described as a pisciform balloon fitted with propelling machinery, inasmuch as the car containing the driving machinery is suspended from the balloon in the manner of the car in the ordinary drifting vessel.