4n PBOGBJ588 <>K PALAEONTOLOGY n 



hand ami with tin- nuuiuantson the other, differed 

 fr.'in both to such an extent that it could find ;i 

 place in neither group. In fact, it held, in some 

 respects, an intermediate position, tending to 

 bridge over tlu- interval between these two groups, 

 which in the existing fauna are so distinct. In 

 tin- same way, the Palccothcrium tended to connect 

 t.'ims so different as the tapir, the rhinoceros, ami 

 the horse. Subsequent investigations have brought 

 to liijht a variety of facts of the same order, tin- 

 most curious and striking of which are those which 

 pro\ \istence, in the mesozoic epoch. of a 



series of forms intermediate between birds and 

 reptiles two classes of vertebrate animals which 

 at present appear to be more widely separated 

 than any others. Yet the interval between them 

 is completely filled, in the mesozoic fauna, by 

 birds which have reptilian characters, on the one 

 side, and reptiles which have ornithic characters, on 

 t lie other. So again, while the group of fishes, 

 termed ganoids, is, at the present time, so distinct 

 from that of the dipnoi, or mudfishes, that they 

 have been reckoned as distinct orders, the 

 Devonian strata present us with forms of which 

 it is impossible to say with certainty whether they 

 are dipnoi or whether they are ganoids. 



Agassiz's long and elaborate researches upon 



fossil fishes, published between 1833 and I ML', 



him to suggest the existence of another kind 



of relation between ancient and modern forms of 





