RANGI: OF FAMILrES IN TIME. 



125 



lY. EANGE OF FAMILIES IN TIME. 



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

 "fam.ilies" for natural groups of genera, and adojDting the 

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

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

 Beven have become extinct, yiz. : — 



Belemnitidae. 

 Ammonitidje. 

 Ortlioceratidae. 



Spiriferidse. 



Orithidae. 



Productidae. 



Hippuritidae. 



