168 PICTURESQUE SKETCHES 



The hinder extremities, although strong, do not 

 exhibit any special contrivance, and were not at- 

 tached with unusual strength to the back-bone and 

 the sacrum, so that it appears that the habits and 

 necessities connecting this creature with the fishes 

 have been here chiefly considered. The hinder ex- 

 tremities are also usually smaller than the others. In 

 serpents we have a similar arrangement ; but in that 

 case the extremities are merely rudimentary, and are 

 of no use to the animal in locomotion. 



The paddles or fins by which the Ichthyosaurus 

 made its way throu'gh the water are very curious 

 and interesting, for they are perhaps more perfect 

 modifications of the true fish structure to the habits 

 of an animal occasionally coming on shore, than any 

 that can be referred to in nature. They at the same 

 time exhibit the true reptilian structure of the 

 animal, and thus show us how perfectly and yet how 

 simply the legs of a quadruped and the fins of a fish 

 are still formed according to a similar law of develop- 

 ment, although intended for such different purposes, 

 and presenting such very different appearances. 



We have already considered the peculiar apparatus 

 by which the paddles are made to rest upon the 

 back-bone by means of a sternum or breast-bone 

 affording a resisting power, but not connected with 

 the ribs. It remains now to consider the form and 

 structure of the fin or paddle itself. 



face upon it. These latter bones again also articulate with the breast- 

 bone, so that there is a double attachment, rendering the whole more secure, 

 and providing a resisting surface, against which the extremities may safely 

 press with the whole weight of the animal, without these parts yielding 

 inwards and pressing against the soft muscular masses. 



