animal, moving rapidly through the sea, and breathing air, must have required great modifications of the fore-leg and foot of the Lizard, to fit it for such cetaceous habits. The extremities were to be converted into fins instead of feet, and as such we shall find them to combine even a still greater union of elasticity with strength, than is presented by the fin or paddle of the Whale. Plate 12, Fig. 1, shows the short and strong bones of the arm (e,) and those of the fore-arm (f, g;) and beyond these the series of polygonal bones that made up the phalanges of the fingers. These polygonal bones vary in number in different species, in some exceeding one hundred; they differ also in form from the phalanges both of Lizards and Whales: and derive, from their increase of number, and change of dimensions, an increase of elasticity and power. The arm and hand thus converted into an elastic oar or paddle, when covered with skin, must have much resembled externally the undivided paddle of a Porpoise or Whale. The position also of the paddles on the anterior part of the body was nearly the same; to these were superadded posterior extremities, or hind fins, which are wanting in the cetacea, and which possibly make compensation for the absence of their flat horizontal tail: these hind paddles in the Ichthyosaurus are nearly by one half smaller than the anterior paddles.[1]
Mr. Conybeare remarks, with his usual acumen, that "the reasons of this variation from the proportions of the posterior extremities of quadrupeds in general, are the same which lead to a similar diminution of the analogous parts in Seals, and their total disappearance in the cetacea, namely, the necessity of placing the centre of the organs of motion, when acting laterally, before the centre of gravity. For the same reason, the wings of birds are placed in the fore part of their body, and the centre of the moving forces given to
- ↑ In the Ornithorhynchus, also, the membranous expansion, or web of the hind-feet, is very much less than that on the fore-foot.
