A BIG FOSSIL BONE. 75 



probably our sun-fish, our tunnies, our sharks, 

 and our devil-fish are each in their way larger 

 than almost any previous fishes one living 

 shark actually attaining a length of forty feet. 

 No fossil bivalve molluscs to my knowledge 

 are as big as the common Mediterranean 

 pinna, or as that giant clam, the tridacna, 

 whose shell is so commonly used as a basin 

 for fountains. In fact, there are only two im- 

 portant groups, the birds and the reptiles, in 

 which extinct species were much larger than 

 existing ones ; and in these two groups the 

 decrease is evidently due to the later supre- 

 macy of the mammalian type. 



Similarly, if we take many comparatively 

 modern lines of descent, we shall find that 

 the horses, the deer, the elephants, and 

 several other now dominant groups of animals 

 have been steadily increasing in size from the 

 earliest epoch of their appearance to the 

 recent period. And among the great extinct 

 creatures, some like the moa and the dodo 

 have only quite recently been killed off; 



