EANGE OF FAMILIES IN TIME. 



125 



lY. EANGE OF FAMILIES IN TIME. 



Distrihution of Families of Shells m Time. — Employing the term 

 "families" for natural groups of genera, and adopting the 

 smallest possible number of them, we find that sixteen, or 

 nearly one-fifth, range through all the geological systems. Only 

 seven have become extinct, viz. : — 



Belemnitidae. 

 Ammonitidje. 

 Orthoceratidae. 



Spiriferidae, 



Orithidae, 



Productidae. 



Hippuritidae. 



