vi.] THE SLATES ON THE ROOF. 131 



well-known, animal, of which specimens may sometimes 

 be seen alive in English aquaria. So perished in the 

 lapse of those same ages, the armour-plated or 

 "Ganoid" fish which Hugh Miller made so justly 

 famous and which made him so justly famous in 

 return appearing first in the upper Silurian beds> and 

 abounding in vast variety of strange forms in the old 

 Eed Sandstone, but gradually disappearing from the 

 waters of the world, till their only representatives, as 

 far as known, are the Lepidostei, or " Bony Pikes," 

 of North America ; the Polypteri of the Nile and 

 Senegal ; the Lepidosirens of the African lakes and 

 Western rivers; the Ceratodus or Barramundi of 

 Queensland (the two latter of which approach Am- 

 phibians), and one or two more fantastic forms, either 

 rudimentary or degraded, which have lasted on here 

 and there in isolated stations through long ages, 

 comparatively unchanged while all the world is changed 

 around them, and their own kindred, buried like the 

 fossil Ceratodus of the Trias beneath thousands of feet 

 of ancient rock, among creatures the likes whereof 

 are not to be found now on earth. And these are but 

 two examples out of hundreds of the vast changes 

 which have taken place in the animal life of the 

 globe, between the laying down of the Cambrian 

 slates and the present time. 



Surely and it is to this conclusion I have been 

 tending throughout a seemingly wandering paragraph 

 surely there has been time enough during all those 

 ages for clay to change into slate. 



And how were they changed ? 



I think I cannot teach my readers this more simply 

 than by asking them first to buy Sheet No. LXXVIII. 



K 2 



