Fig. 33.—The Porpoise (Phocœna communis). Here the tail is principally engaged in swimming, the anterior extremities being rudimentary, and resembling the pectoral fins of fishes. Compare with fig. [30], p. 65.—Original.
Fig. 34.—The Manatee (Manatus Americanus). In this the anterior extremities are more developed than in the porpoise, but still the tail is the great organ of natation. Compare with fig. 33, p. 73, and with fig. [30], p. 65. The shape of the manatee and porpoise is essentially that of the fish.—Original.
It is otherwise with the Rays, where the hands are principally concerned in progression, these flapping about in the water very much as the wings of a bird flap about in the air. In the beaver, the tail is flattened from above downwards, as in the foregoing mammals, but in swimming it is made to act upon the water laterally as in the fish. The tail of the bird, which is also compressed from above downwards, can be twisted obliquely, and when in this position may be made to perform the office of a rudder.
Fig. 35.—Skeleton of the Dugong. In this curious mammal the anterior extremities are more developed than in the manatee and porpoise, and resemble those found in the seal, sea-bear, and walrus. They are useful in balancing and turning, the tail being the effective instrument of propulsion. The vertebral column closely resembles that of the fish, and allows the tail to be lashed freely about in a vertical direction. Compare with fig. [29], p. 65.—(After Dallas.)
Swimming of the Seal, Sea-Bear, and Walrus.—In the seal, the anterior and posterior extremities are more perfect than in the whale, porpoise, dugong, and manatee; the general form, however, and mode of progression (if the fact of its occasionally swimming on its back be taken into account), is essentially fish-like.