GEOGRAPHIC DISTRIBUTION OF GYMNOSPERMS 179 



monotypic Tumboa is narrowly restricted to certain extremely 

 arid regions of western Africa. 



There is nothing in the geographical distribution of this 

 group to suggest anything in common, for the range and condi- 

 tions are as diverse as are the structures. If they are genet- 

 ically related, they must represent a very ancient and once 

 widely distributed phylum, but no sure evidence of its presence 

 has been discovered among fossil plants. 



