140 On Equisetum in the Yorkshire Coal-measures. 
the fossil, also enlarged, whose similar characters are observ- 
able. To return to fig. d, the central tubercle indicates the 
point at which the peltate-shield is attached to its stalk ; 
hence there is a greater thickness of tissue at this part, and in 
drying the peripheral portions of the shield shrink more, and, 
being unsupported underneath, fall below the level of the 
centre, and thus the central tubercle is formed. One of the 
peltate-shields is shown in profile at fig. e *, which illustrates 
more fully their structure. 
a, Eyuisetum Hemingwayi, Kidston, Nat. size. 
b, Outer surface of one of the sporangiferous shields of LEguesetum Hem- 
ingwayt. Enlarged. 
c. Cone of Equisetum limosum, 8m. xX 43; from a photomicrograph. 
d. Outer surface of a sporangiferous shield of Equisetum limosum. En- 
larged. 
e. Sporangiferous shield of existing Zgursefwm seen in profile, to show 
central column and sporangia. Hnlarged. 
Now though the internal structure of the fossil cones is 
unknown, I think we are quite warranted in supposing that 
the central tubercle and slightly elevated margins of the plates 
* From Maout and Decaisne, ‘ General System of Botany,’ English ed. 
p. 904 (1873). 
