LIFE ON THE EAETH. 79 



have not been certainly recognized, all the important 

 classes of Marine Invertebrata are traced into the 

 Lower Paleozoic Strata, beginning in each case with 

 few species and very few genera. The progress of 

 the several classes is very unequal. Crustacea, rela- 

 tively abundant in every stage, reach a maximum in 

 the third period. Brachiopoda, also a very abundant 

 group, reach the maximum in the fifth period with 

 Zoophyta and Echinodermata, while the Monomyaria, 

 Dimyaria, Gasteropoda, and Cephalopoda increase, 

 though not uniformly, upwards to the sixth period. 

 The seventh period is everywhere marked by a zone 

 of sterility, the local extinction of most of the classes, 

 and the introduction of a new order of sediments, 

 brought by a new set of watery currents. Arranging 

 the classes according to their priority of appearance, 

 including Fishes, and giving to each in the successive 

 strata a space proportioned to the number of species, 

 we construct the scheme of proportionate life for the 

 Lower Palaeozoic Strata, represented in Fig. 5. 



