416 MANUAL OF THE MOLLUSCA. 



Distribution of Families of Shells in Time. Employing the term "fami- 

 lies " for natm-al groups of genera, and adopting the smallest possible number 

 of them, we find that sixteen, or nearly one-fifth, range through all the geolo- 

 gical systems. Only seven have become extinct, viz. — 



Belemuitidae. Spiriferidae. Hippuritidse. 



Ammouitidae. Ortbidse. 



Orthoceratidae. Productidae. 



Three others are nearly extinct : — 



Nautilidae. Ehyuchonellidae. Trigoiiiadse. 



And several have passed their maximum, and become less varied and abundant 

 than formerly, e.g. — 



Tomatellidae. Cyprinidas. Anatinidae. 



The extinct families and genera appear to have attained their ma.tima 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 Palseozoic 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 Tertiaries 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 Silm-ian period ; and since 

 there is no evidence or indication that the earth was ever destitute of life, 

 either wholly or in part, it foDows almost as a matter of necessity that the 

 early types must have been more widely distributed and individually deve- 

 loped, 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 discover^/ 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 Paleozoic genera ; and the establishment of 

 fresh and arbitraiy distinctions 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 shev/n 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. 



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