ICHTHYOSAURUS. 
183 
required great modification 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 be- 
yond these the series of polygonal bones tliat 
niade 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, wlien 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 super- 
added 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 Ich- 
thyosaurus are nearly by one half smaller than 
the anterior paddles.* 
In the Ornithorhynchiis, also, the membranous expansion, or 
Web of the hind feet, is very much less than that on the fore foot. 
