76 LIFE ON THE EARTH. 



Echinodermata 20 genera 34 species. 



Annelida 4 6 



Crustacea 16 36 



Polyzoa 12 25 



Brachiopoda ... 17 79 



Monomyaria ... 2 12 



Dimyaria 7 15 



Gasteropoda ... 8 18 



Heteropoda 1 3 



Pteropoda 2 3 



Cephalopoda ... 3 22 



The prevalence of Zoophyta is connected with 

 the great abundance of limestone in this Zone, a 

 circumstance less favourable to Crustaceans, though 

 these are still numerous ; all Entomostracous, mostly 

 Trilobites. The Monomyarians are all Aviculoid, the 

 Dimyarians mostly Arcaceee and MytilacesG. 



In the Ludlow Zone which follows, we have to 

 remark a general analogy to the Caradoc deposits, 

 both being for the most part arenaceous and muddy 

 deposits, with only partial expansions of the calca- 

 reous element. Hence fewer Zoophyta and Echino- 

 dermata, more Monomyaria and Dimyaria, these bi- 

 valves being still of the same natural families. In 

 the uppermost layer Land-plants and Fishes appear 

 for the first time in the History of Life. 



Plants probably terrestrial 1 genus 1 species. 



marine... 2 genera 5 



Amorphozoa (none) 



