92 THE GENESIS OF SPECIES. [CHAP. 



Thus the great oceanic Mammalia the whales show 

 striking resemblances to those prodigious, extinct, marine 



SKELETON OF AN ICUTHYOSAUEU8. 



reptiles, the Ichthyosauria, and this not only in structures 

 readily referable to similarity of habit, but in such matters 

 as greatly elongated premaxillary bones, together with the 

 concealment of certain bones of the skull by other cranial 

 bones. 



Again, the aerial mammals, the bats, resemble those fly- 

 ing reptiles of the secondary epoch, the pterodactyls; not 

 only to a certain extent in the breast-bone and mode of sup- 

 porting the flying membrane, but also in the proportions of 

 different parts of the spinal column and the hinder (pelvic) 

 limbs 



Also bivalve shell-fish (i. e., creatures of the muscle, 

 cockle, and oyster class, which receive their name from the 

 body being protected by a double shell, one valve of which 

 is placed on each side) have their two shells united by one 

 or two powerful muscles, which pass directly across from 

 one shell to the other, and which are termed " adductor 

 muscles " because by their contraction they bring together 

 the valves and so close the shell. 



Now there are certain animals which belong to the crab 

 and lobster class (Crustacea) a class constructed on an 

 utterly different type from that on which the bivalve shell- 

 fish are constructed which present a very curious approxi- 

 mation to both the form and, in a certain respect, the 

 structure of true bivalves. Allusion is here made to certain 



