140 On Equisetum in the Yorkshire Coal-measures. 



the fossil, also enlarged, whose sunilar characters are observ- 

 able. To return to fic". 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 slirink 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. Equisetum Heminfftva;/i, Kidston. Nat. t^ize. 



b. Outer surface of one of the spoi-angiferous shields of Equi<etum Uem- 



higwayi. Enlarged. 



c. Cone oi Equifetuni liuiosum, Sm. X 4i ; from a pliotoniicrograph. 



d. Outer surface of a sporangiferous shield of Equisetum limoswi. En- 



larged. 



e. Sporangiferous shield of existing Equisetum set-n in profile, to show 



central column and sporangia. Enlargt^d. 



Now though the internal structure of the fossil ernes is 

 unknown, 1 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 IJutanv." English eJ. 

 p. 904 (l«7:Jj. 



