54 AN INTRODUCTION TO THE STUDY OF FOSSILS 



of Lepidostrobus, and usually occur as impressions in coal; 

 the internal structure is well known from the study of the cal- 

 cified specimens of the coal balls 

 together with several silicified 

 specimens and casts. The cones 

 vary from an inch to one and a 

 half feet in length. 



Sigillaria (Figs. i6, 150). — This 

 was a tree which probably re- 

 sembled Lepidodendron in general 

 appearance as well as in geologic 

 range. It differed in the structure 

 of its stem and in the arrangement 

 of leaves, as is indicated by the 

 impressions of the bark covered 

 with leaf-cushions. The leaf- 

 cushions are hexagonal in out- 

 line and arranged in quite regu- 

 lar vertical rows. Its cones are 

 known as Sigillariostrobus. 

 Its underground stems have been discussed under Lepidoden- 

 dron. 



1. Name tw^o common living club-mosses. Where have you 

 seen them growing ? 



2. What is the geologic range of this order ? 



3. Sketch a leaf-cushion of Lepidodendron; label, where 

 present, scar left by fall of leaf, the decurrent base of leaf, and 

 of vascular bundle passing into leaf. 



4. Sketch (reduce in size) a foot length of Stigmaria, a small 

 portion in detail, indicating the little ''roots." What was the 

 function of Stigmaria? 



5. Sketch a small portion of Sigillaria. How does it differ 

 from Lepidodendron ? 



6. What are coal balls? In what country and in what 

 geological formation do they occur most abundantly ? 



7. Why are the fruit, stem and roots of Lepidodendron and 

 Sigillaria known under separate names ? 



8. How do fossil lycopods resemble living ones ? 



Fig. 16. — Sigillaria polila Lesque- 

 reux, from the Pennsylvanian 

 formations of Pennsylvania, show- 

 ing the leaf scars arranged in 

 vertical rows. Natural size. 

 (From Lesquereu.x.) 



