126 MANUAL OF THE MOLLTTSCA. 



Throe others are nearly extinct : — 



Nautilidae. Rhj'nchonelliclae. Trigoniadse. 



And several have passed their maximum, and become less varied 

 and abundant than formerly, e.g. — 



Tornatellidae. Cyprinidae. Anatinidse. 



The extinct families and genera appear to have attained thei;. 

 maxima more rapidly than their minima ; continuing to exist., 

 under obscure forms, and in remote localities, long after the 

 period in which they flourished. 



The introduction of new forms, also, is more rapid than the 

 process of extinction. If four Palaeozoic families disappear, 

 twenty-six others replace them in the Secondary series ; and 

 three of the latter are succeeded by fifteen shell-bearing families 

 in the Tertiary and existing seas. 



In consequence of this circumstance, the number of types is 

 three times greater in the newer Tertiary than it was at the 

 Silurian period; and since there is no evidence or indication 

 that the earth was ever destitute of life, either wholly or in 

 part, it follows almost as a matter of necessity that the early 

 types must have been more widely distributed and individually 

 developed, than those of the present day. 



From the following Table it will be seen that the number of 

 genera and families increases with an amount of regularity 

 which cannot be accidental. Moreover, the relation of these 

 numbers is not liable to be much altered by the progress of 

 discovery or the caprice of opinion. The discovery of new types 

 is not likely to be frequent ; the imposition of new names, in 

 place of the old, will not increase the number of Palaeozoic 

 genera ; and the establishment of fresh and arbitrary distinc- 

 tions will affect all the groups in due proportion. 



If the number of groups called " Systems " were reduced to 

 seven (viz., three Palaeozoic, three Secondary, and one Tertiary, 

 as shown in the following Table), then the average duration of a 

 genus of shells would be equal to a System of Formations. 



The duration of the smallest well-defined Families of shells 

 is about equal to one of the three great Geological Divisions, 

 or Ages. 



