PALEOBOTANY BEERY. 



321 



to 10 homosporous sporangia. The rhizomes or underground stems 

 of the species with annual shoots commonly form tubers, and these 

 often occur abundantly as fossils, as in the Lower .Cretaceous of 

 Maryland. The Equisetalcs were represented by a number of small 

 and imperfectly known Paleozoic species referred to the genus Equi- 

 setites. During the Triassic they were represented throughout the 

 world by stem casts, often of large size, with alternating ribbed 



Fig. 10. — Diagram showing the geologic history and phylogeny of the Arthrophyta. 



stems and the leaves united into sheaths at the nodes. They con- 

 tinued as not uncommon types throughout the Jurassic in all parts 

 of the world, decreasing gradually in size and abundance during 

 the later Mesozoic until their status and size in the Cenozoic floras 

 was much as in the existing flora. 



The generalized geological ranges of the various arthrophyte 

 groups as well as their relative importance at different periods and 

 their filiation are shown in the accompanying diagram. 



