LIFE ON THE EARTH. 87 



Crinoidea are very rare in a living state, Echinoidea 

 plentiful. Now 29 species of Crinoidea occur in the 

 Lower Palaeozoic, 15 in Middle Palaeozoic, and 105 in 

 Upper Palaeozoic Strata. After this they grow com- 

 paratively rare, though still 29 species occur in Me- 

 sozoic Strata, and 5 in the Caenozoic series. One 

 recent species ! On the other hand, Echinoidea are 

 represented in the Palaeozoic series by 12 species, in 

 the Mesozoic by 173, and in the Caenozoic by 25. 



All the Echinoidea of the Palaeozoic series belong 

 to the regular division, with the openings of the 

 alimentary canal opposite (E. endocyclida). The 

 same group occurs in the Mesozoic Series, but in ad- 

 dition we have a second equally large group with 

 these openings not opposite (E. exocyclida), and this 

 is continued in the modern ocean. 



The Echinoidea of the Palaeozoic Series have 

 such peculiarities in the series of plates and pores 

 as to claim to be enrolled in separate families (Pa- 

 laechinidae and Archaeocidaridae), or even to consti- 

 tute an order (Perischoechinoidea) equal to that of 

 the Echinoidea. 



The contrasts which have thus been stated be- 

 tween the Palaeozoic and later forms of Radiated 

 Animals may be represented in the following tabular 

 view; where the word Neozoic is used to include 

 Mesozoic and Caenozoic ages. 



