1867.] 



^ Thesaurus Siluricus.' 



383 



The orders vary greatly in respect to recurrency. There is none among 

 fossil fish. In Cystidea it is only 3 per cent., in Gomphoceras 5 per cent., 

 and is greatest in Strophomena, 31 per cent. 



Although a considerable number of inferences have been prepared, I 

 shall only venture now to introduce a few. 



1 . Recurrence is universal, both as to time and place. 



2. Recurrences seem to be most numerous in the lower stages of the 

 epoch ; but further research may teach otherwise. 



3. Species do not often change their horizon, not even when placed in 

 countries far apart. 



4. The same species may be typical of a single horizon in one country 

 and recurrent in another. 



5. Recurrency shows that a mollusk is not necessarily confine^ 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 con- 

 ditions. 



7. Communities, genera, and species disappear sporadically, except 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 pre? 

 vailing than the reduction by Mr. Salter to 133 of the 439 palaeozoic spe- 

 cies which I had tabulated as extra-epochal, although they had the sanc- 

 tion of the best palseontologists 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My Table, as originally made out, deals with the five palaeozoic epochs, 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 interesting additions have 

 been made. 



I. These recurrents are mostly distinct from the intra-epochal, owing to 

 their first appearance being in the Upper Silurian stage. 



II. With the exception ofChonetes sarcimdata, they all stop within the 

 Devonian Period. 



III. The greater part of these recurrents are of low rank ; 20 are 

 Brachiopoda ; 1 1 Zoophytes, 1 an Amorphozoa ; 7 are Gasteropoda ; 3 

 Cephalopoda ; and 1 Trilobite. Manon deforme and Orthis rugosa, Lower 

 Silurian fossils, reappear in Devonian, but not in Upper Silurian, where 

 they are presumably " — to use an expression of Mr. Etheridge. 



ly. These species are very migratory — few being found in two epochs 

 in the same country, but in different countries. 



V. Opportunities of escape into a new epoch have been common ; but 

 the ways and means are frequently concealed by denudations, &c. 



VI. Acclimatization must have been necessary. 



VII. The length of individual life in proportion to specific extra- 

 epochal life is almost as a unit to infinity. 



