DESCRIPTION OF THE SPECIES. 



CRYPTOGAMS. 



EQUISETEvE. 



EQUISETUM L. 



None of tlie Equisefece except Equisetum have been found in tlie flora 

 of the lower Potomac. Neither of the species of Scltlzoncnra, wliich occur 

 in the flora of the older Mesozoic, or RluTjtic of the Richmond Coal Field, 

 appears. As compared with the si)ecies of Equisetum of the older i\Ieso- 

 zoic. those of the Potomac formation show a marked change. In the older 

 Mesozoic the variety of forms, it is true, is small, but the number of indi- 

 viduals of Equisetum Moqersi is very great, and the specimens are often to 

 be found filling certain strata in such numbers as to show that the plants 

 must have grown thickly over large areas, and nuist have tbrmed the 

 greater part of the vegetation of the nuu-sh. The great size attained by all 

 of these individuals and the rarity of their branching show that they 

 belong to the Triassic and older Jurassic t3'pe. 



The species of the Potomac strata show decidedly more modern 

 aspects, and clearly Ijelong in type to the younger Jurassic and Wealden, 

 showing marked affinity with the species of these periods, especially the 

 latter. Specimens are rare, and they occur singly, scattered very spai'ingly 

 in certain strata. The plants were small and mostly much branched. In 

 these points they resemble living forms. 



Equisetum Virginicum, sp. nov. 



Plate I, Figs. 1-6, 8; Plate II, Figs. 1-3, 6, 7, 9. 



Stems and liranches branch copiously ; the diameter ranges from 3""" 

 to 8°"°, the average being -i'"""; length of internodes, 8'"'" to 22"""; sheaths 



