126 MANUAL OF THE MOLLUSCA. 
Three others are nearly extinct :-— 
Nautilidse. Rhynchonellide. Trigoniade. 
And several have passed their maximum, and become less varied 
and abundant than formerly, e.g.— 
Tornatellide. Cyprinide. Anatinide. 
The extinct families and genera, appear to have attained thei_ 
maxima tnore rapidly than theiz minima; continuing to exist, 
under obscure forms, and in rezeote localities, long after the 
period in which they fiourished. 
The introduction of new forms, also, is more rapid than the 
process of extinction. If four Paleeozoic families disappear, 
twenty-six others repiace them in the Secondary series; and 
three of the latter are succeeded by fifteen shell-bearing families 
in the Tertiary and existing seas. 
In ponsoaucace of this circumstance, the number of ieee 18 
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 hfe, 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 Paleozoic 
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 
seyen (viz., three Palzeozoic, 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. 
