PART II. 



THE FOSSIL FLORA. 



DESCRIPTION OF THE SPECIES. 



EQUISETE^. 



EQUISETUM, L. 



Equisetum Rogers!, Schimper. 



Plate I, Fig. 2; Plate II, Figs. 1 and 2. 



Stem 6 to 9 centimeters thick, furrowed below the sheath for about 1£ centime- 

 ters, the rest of the internode smooth. Lower internodes shorter than the upper, with 

 the length gradually increasing in ascending. Sheaths closely appressed, and 12 to 15 

 millimeters long. Teeth, 70 to 80 in number, about 8 millimeters long, ribbed, linear, 

 and narrow to near the base, where they rapidly expand into the summit of the united 

 leaves or ribs. Ribs, or united portions of sheath leaves, linear and separated from 

 one another by a sharply- distinct keeled furrow, concave on the back, the concavity 

 being embraced within two sharply-defined raised lines, which at the base of the rib 

 lie at its outer margins, but gradually approach each other towards the summit of the 

 same. At the summit of the rib they pass into the teeth and soon become approxi- 

 mately parallel, being almost in contact, forming the rib of the tooth. 



The above-mentioned raised lines on the back of the ribs are the most 

 characteristic feature in the sheath of this Equisetum. They begin, as 

 stated, at the base of the rib, on its outer margins, and here the close prox- 

 imity of the similarly-placed line on the adjoining rib determines a depression 

 between the two ribs, whose cross-section is an acute angle, with its apex 

 downwards. In ascending towards the summit of the rib the two lines 

 approach each other, and depart more and more from the margin, so that 

 the concavity of the back of the ribs, which is caused by the inward slope 

 from these raised lines, and which is very slight near the base of the ribs, 

 where they are far apart, becomes quite pronounced at the summit, where 



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