102 LIFE ON THE EARTH. 



Having so limited a total range in time, but 



. including some hundreds of well-marked specific 



; for, ws, a^d bejng widely diffused in geographical space, 



Ammonites furnish to the Palaeontologist admirable 



data for fixing the chronological succession of the 



secondary strata, in cases when these strata are not 



continuously traceable. Taking some well-known 



forms, including Ceratites, and placing them in the 



order of their superposition, we obtain the following 



series of geological epochs in the great Mesozoic 



Period. 



No Ammonites or Ceratites or Goniatites in Caenozoic Strata. 



f Amm.Rhotomagensis Lower Chalk. 



A. varians Upper Green Sand. 



Cretaceous Period { lu 



A. auritus Gault. 



[A. Deshaysii Lower Green Sand. 



UpperOolitio Age ( A ' ^ ntens Portland Oolite. 



\ A. biplex Kimmendge Clay. 



( A. vertebralis . . . .Oxford Oolite.and below. 

 Middle Oolitic Age I. _ .. ' . 



\ A. calloviensis Kelloways Rock. 



A. macrocephalus Cornbrash, and below. 



A. gracilis Great Oolite. 



A. Parkinsoni Inf. Oolite upper part. 



A. Humphreysianus ...Inf.Oolite middle part. 

 A. Murchisonae Inf. Oolite lower part. 



83 



II 



Lower Oolitic Age 



P 



A. Jurensis Sand. 



{A. bifrons Upper Lias. 

 A. Conybeari Lias Limestone. 

 A. planorbis Lowest Lias. 



Triassic Period Ceratites nodosus Muschelkalk. 



No Ammonites or Ceratites in the Palaeozoic Strata. 



