IX GEOLOGICAL CONTEMPORANEITY S()l 



criticism than these ; and if this be so, no careful 

 reasoner would, I think, be inclined to lay very 

 great stress upon them. Among the Vertebrata, 

 however, there are a few examples which appear to 

 be far less open to objection. 



It is, in fact, true of several groups of Verte- 

 brata which have lived through a considerable 

 range of time, that the endoskeleton (more par- 

 ticularly the spinal column) of the older genera 

 presents a less ossified, and, so far, less differ- 

 entiated, condition than that of the younger 

 genera. Thus the Devonian Ganoids, though 

 almost all members of the same sub-order as 

 Polypterus y and presenting numerous important re- 

 semblances to the existing genus, which possesses 

 biconclave vertebrae, are, for the most part, wholly 

 devoid of ossified vertebral centra. The Mesozoic 

 Lepidosteidae, again, have, at most, biconcave 

 vertebrae, while the existing Lepidosteus has 

 Salamandroid, opisthocoelous, vertebrae. So, none 

 of the Palaeozoic Sharks have shown themselves 

 to be possessed of ossified vertebrae, while the 

 majority of modern Sharks possess such vertebrae. 

 Again, the more ancient Crocodilia and Lacertilia 

 have vertebrae with the articular facets of their 

 centra flattened or biconcave, while the modern 

 members of the same group have them procoelous. 

 But the most remarkable examples of progressive 

 modification of the vertebral column, in corre- 

 spondence with geological age, are those afforded 



