300 



Royal Society. 



to any one community, but may find a home and flourish in several 

 successively. 



6. The number of recurrents measures the amount of change in 

 conditions. 



7. Communities, genera, and species disappear sporadically, ex- 

 cept in the rare case of a catastrophe. 



8. Recurrency is a measure of viability. 



Extra-epochal Recurrence. 



Few things demonstrate more plainly the sterner discipline now 

 prevailing than the reduction by Mr. Salter to 133 of the 439 palaeo- 

 zoic species which I had tabulated as extra-epochal, although they 

 had the sanction of the best palaeontologists of the last fifteen or 

 twenty years. 



My Table, as originally made out, deals with the five palaeozoic 

 epochs, but in this place only with the forty-two Silurian species 

 which leave for the higher periods. To these, recently, several inter- 

 esting additions have been made. 



Table F. — Geographical Summary of Silurian Life. 



Orders. 



Plantae 



Amorphozoa 



Rhizopoda 



Coelenterata 



fg ^ r Crinoidea 



g "e8 j Cystidea 



'•% S [ Asteriada 



Annelida 



i ^ fTrilobitae 



g I -^ Phyllcpoda 1 

 O -^ [ Ostracoda J ' ' 



Polyzoa 



Brachiopoda 



Monomyaria 



Dimyaria 



Pteropoda & Heteropoda 



Gasteropoda 



Cephalopoda 



Pisces 



Incertae Sedis 



4 



262 



193 

 56 

 29 



36 

 396 



77 



203 

 678 

 78 

 181 

 103 

 421 

 321 



3156 



56 20 

 62 63 

 ?25 

 245 



170 



177 

 721 



56 

 241 

 145 

 274 

 861 



34 

 2 



4305 



10 



< 



57 ... 100 



43 



231 



* Tibet. 



To America and Europe. 

 Not definitely accepted. 

 To America and Europe. 



Various. 



Various. 

 Various. 

 To America and Europe. 



3156-}- 4r05=^ 7461 species. 



