5 66 



HISTORICAL GEOLOGY 



generally short and stout, never apparently reaching a greater height 

 than three or four feet, and usually much less. As in modern cycads, 

 a crown of long, stiff leaves sprang from the top of the trunk, which was 

 scarred throughout by the leaf-bases of previous leaves. Some 

 fossil cycads from South Dakota are so completely preserved that such 

 delicate structures as immature leaves, flowers, pollen, and some seeds 

 with their contained embryos, are retained in remarkable perfection. 



Fig. 526. — A group of cycad trunks (Bennettites). (After Wieland.) 



Because of this perfection of preservation, almost as much is known 

 of the structure of this extinct group as of its living relations. The 

 position of the seed-bearing cone and the large leaves bearing the pol- 

 len sacks of a fossil cycad is well shown in the diagram (Fig. 527). 

 Cycads, which appear to have grown on the dryer lowlands about 

 the swamps, had their origin in the Permian, reached their greatest 

 abundance in the Jurassic, and are rare after the close of the 

 Mesozoic. 



Ferns were common throughout the era, wherever the conditions 

 were favorable for their growth. 



