302 GEOLOGICAL CONTEMPORANEITY ix 



by the Pycnodonts among fish, and the Labyrin- 

 thodonts among Amphibia. 



The late able ichthyologist Heckel pointed out 

 the fact, that, while the Pycnodonts never possess 

 true vertebral centra, they differ in the degree of 

 expansion and extension of the ends of the bony 

 arches of the vertebrae upon the sheath of the 

 notochord ; the Carboniferous forms exhibiting 

 hardly any such expansion, while the Mesozoic 

 genera present a greater and greater development, 

 until, in the Tertiary forms, the expanded ends 

 become suturally united so as to form a sort of 

 false vertebra. Hermann von Meyer, again, to 

 whose luminous researches we are indebted for 

 our present large knowledge of the organisation of 

 the older Labyrinthodonts, has proved that the 

 Carboniferous Archegosaurus had very imperfectly 

 developed vertebral centra, while the Triassic 

 Mastodonsaurus had the same parts completely 

 ossified. 1 



The regularity and evenness of the dentition of 

 the Anoplotherium, as contrasted with that of exist- 

 ing Artiodactyles, and the assumed nearer approach 

 of the dentition of certain ancient Carnivores to 

 the typical arrangement, have also been cited as 

 exemplifications of a law of progressive develop- 

 ment, but I know of no other cases based on 



1 As this Address is passing through the press (March 7, 

 1 862), evidence lies before me of the existence of a new Laby- 

 rinthodont (Pholidogaster), from the Edinburgh coal-field with 

 well-ossified vertebral centra. 



