THE CRUST OF THE EAETH. 



genera have been found in all the rocks from the Silurian 

 upwards, and in gradually- increasing numbers. They were, 

 therefore, among the earliest manifestations of animal life on the 

 globe; and what is remarkable is, that most of the genera, 

 including even those of the Silurian period, still survive. 



The flose analogy of these ancient forms with the existing 

 species will be manifest, By some examples taken from among the 

 countless numbers of fossil shells collected by geologists. 



A fossil shell from the Permian group is shown in fig. 12, and 

 one found in all the tertiary beds, in fig. 13. 



One of the genera which first appears in the middle strata of 



Fig. 14. Voluta Egata. 



the cretaceous group is shown in fig. 14, and one which begins in 

 the middle of the oolite, in fig. 15. 



Fig. 15. Pterocera Oceani. 



