LAW OF PROGRESS AND APPEARANCE OF GROUPS 247 



upper Silurian to the end of Primary times ; recent 

 discoveries in the canons of Montana push back 

 this lower limit to the pre-Cambrian. 



The palaeontological history of the Vertebrates 

 will yield us still more striking examples of this 

 successive pushing back of the date when several 

 groups first appeared on the earth. When Mur- 

 chison discovered in the last strata of the 

 upper Silurian of England, the fin-spines and 

 teeth of Selachians, mingled with the dermic 

 shields of Placoderms, it was for a long time thought 

 that we were dealing with the earliest Fishes of all. 

 But here again North America has disclosed to us 

 the existence, as early as the Ordovician stage, of 

 numerous remains of Ganoid Fishes which open 

 out horizons of still earlier ichthyological faunas 

 in the Cambrian or the pre-Cambrian. We know 

 now, thanks to a lucky discovery by Lohest, 

 that the Amphibians already existed at the epoch 

 of the formation of the schists of the Fammene, that 

 is, in the upper Devonian. The true Reptiles were 

 long supposed to have appeared only at the com- 

 mencement of Secondary times. Several orders, 

 of marine or terrestrial habitat, of this class, have 

 been discovered later in palaeozoic find-spots. Among 

 the most ancient types the group of the Rhyncho- 

 cephalians should be noted, with a lacertiform body, 

 bi-concave vertebrae, and a breastplate of highly 

 developed ventral ribs, of whom only one existing 

 representative, the genus Hatteria, dwells on the 

 coasts of the New Zealand Archipelago. Credner 

 has shown us that a species very nearly akin to the 

 existing type, the Palceohatteria, already lived in 



