246 THE TRANSFORMATIONS or THE ANIMAL WORLD 



entered as the last actor upon the changing stage 

 of the world. 



Still we are compelled to acknowledge that the 

 seeming regularity of this picture has been rather 

 seriously affected by the latest palseontological 

 discoveries. The Molluscs were considered but 

 a few years ago as having first appeared in the 

 Tremadoc stage at the borders of the Cambrian and 

 the Silurian. Walcott has described a tiny Lamelli- 

 braneh, Modioloides, in the lower Cambrian, and 

 he has just pointed out some patelloid shells of 

 the genus Chuaria in the pre-Cambrian of the 

 Rocky Mountains. The Cephalopoda, who are 

 the highest organized Molluscs, were for a long 

 time only known from the Ordovician stage on- 

 ward. There have now been found in the Cambrian 

 of Esthonia and Nova Scotia straight - shelled 

 Nautilidse of the genus Volborthella. The true 

 Ammonitidae, with slashed and speckled partitions, 

 were long considered as special to the Secondary 

 times : but palaeontologists in India, in the Ural, in 

 Sicily, and in the Pyrenees have revealed to us 

 their presence in the lower Permian, and the ex- 

 istence, at this level, of manifold branches, giving 

 us a glimpse of a still earlier ancestry. The ap- 

 pearance of natatory Crustacea of the order of 

 Trilobites, after having been pushed back from the 

 Ordovician to the middle Cambrian, and then to 

 the lower Cambrian, has been also noticed in 

 the pre-Cambrian of North America. It is the 

 same with another order of great marine Crustacea, 

 the Gigantostraca, or Eurypterids, of which the 

 stratigraphic extension seemed limited from the 



