LIFE ON THE EARTH. 



55 



by a few examples of living species selected from 

 marine animals only 1 . 



But if we examine the fossil and living races by 

 genera instead of species, the result is different. 

 For instance, some existing genera of Mollusca occur 

 in the Mesozoic, and perhaps even in the PalsDozoic 

 Strata, though in these latter the genera Avicula, 

 Modiola, Nucula, &c., among Conchifera, are by 

 some writers thought to be distinguishable from 

 their living analogues. 



The subjoined table is meant to illustrate this 



1 The asterisk indicates in what deposits the living species 

 occur. E. Echinodermata ; A. Annelida ; Cr. Crustacea ; P. Poly^ 

 zoa or Bryozoa ; B. Brachiopoda ; M. Monomyaria ; D. Dimyaria j 

 G. Gasteropoda ; Cet. Cetacea. 



