xiv. CHANGE OF THE FORMS OF LIFE. 399 



Shotover, or the oolites of Cheltenham, Cumnor, and Swindon, 

 we observe in each of the three examples characteristic differences in 

 the organic forms, it will be reasonable to refer these, at least 

 in the first instance, to the difference of the time ; in other words, 

 the forms have changed with the lapse of time. 



That we do find characteristic specific differences is matter of 

 universal consent. Thus, to take the clays, we have in the three 

 great deposits of this nature 



Ostrea deltoidea. Ammonites biplex . . . Kimmeridge clay. 

 Gryphaea dilatata. Duncani . . Oxford clay. 



,, incurva. bifrons . . . Lias. 



And, to take the limestones, we have 



Trigonia gibbosa. Ammonites gigas . . Portland oolite. 



clavellata. perarmatus . . Coralline oolite. 



,, striata. Parkinson! . . Inferior oolite. 



(The sands have but a restricted fauna, and contain few species 

 not found in the limestones and clays.) 



Many species and many genera of fossil marine animals appear 

 to have their beginning, progress, and end discoverable by ob- 

 servation ; and these phases of each life are often completed in 

 one great system of strata, a species often lasting through 50, 

 100, or more feet of deposits, a genus through 1000 or more feet. 

 Thus Terebratula fimbria, in the middle part of the Inferior oolite, 

 and T. coarctata, in the Bradford clay and upper layers of Bath 

 oolite, are almost limited to those narrow zones, while the genus 

 which includes them began its long career before the oolitic system, 

 and is still in existence. In Plates V. and VI. certain zones of 

 characteristic species of cephalopods are marked on the -scale 

 of the oolitic and cretaceous systems. A very large proportion 

 of the genera of marine oolitic invertebrata is found fully re- 

 presented in the Bath group : the number diminishes each way, 

 there being fewer in the lias below, and fewer in the oolites above. 

 Hence arises the idea of the oolitic fauna having its capital, so 

 to speak, in the Bath oolites. Originating in the lowest lias, it 

 attained its maximum of extension in the zone of those oolites, 

 and then declined to extinction in the uppermost layers. 



The same idea presents itself to the student of palaeozoic life, 

 especially the large Cambro- Silurian series, which, beginning with 

 a few scattered species, grows to great richness in the Bala and 



