242 



RANGE OF FAMILIES IN TIME. 



Systems 

 of Strata. 



Pectinidse, 



Avicnlidse — MytilidiB, . . 

 Arcadse^ — ^Trigoniadse, . . 



TJnionidi3e, 



Chamidee — Mj^adse, . . . 



Hippuritidse, 



Tridacnidae, 



Cardiad^ — Lucinidse, 



C^'Cladides, 



Cyprinidee — Anatinidae, . . 



Astartidas, 



Veneridse — Tellinidse, . . 



Mactridae, 



Solenidas, 



Gastrochaenidae — Pholadidae, 



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Three families are nearly extinct : — 



Nautilidas. Rhynehonellidae. Trigoniadae. 



And several have passed their maximum, and become less varied 

 and abundant than formerly, e. g. — 



Tornatellidae. C3q3rinidae. Anatinidae. 



The extinct families and genera appear to have attained their 

 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 Palseozoic families disappear, 

 twentj^'-six others replace them in the Secondary series ; and 

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

 in tl\e 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 devel- 

 oped, 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 dis- 



