24 EQUISETITES. 
So far as facts are available, we are warranted in the conclusion 
that the species of the genus Hyuisetum, from the Upper Carboni- 
ferous period to the present day, appear to have undergone but 
little change in habit and external appearance; the most striking 
difference being one of size. In the Triassic period, when 
Equisetites seems to have reached its maximum development, we 
find an abundance of very large stems; from that time the size 
gradually decreases, and, in the Wealden strata, the few species 
that are known approach much more nearly in size to the living 
members of the family. 
1.—Equisetites Lyelli, Mantell. 
1833. Hquisetum Lyelli, Mantell, Geol. S.E. England, p. 245, figs. 1-3. 
1845. Eguisetites Lyelli, Unger, Gen. spec. plant. foss. p. 60. 
1848. Equisetites Lyelli, Bronn, Index Pal. Nomencl. p. 464. 
1849. Equwisetum Lyelli, Brongniart, Tableau, p. 107. 
1854. Equisetites Lyelli, Morris, Brit. foss. p. 8. 
1869. Hquisetwm Lyelli, Schimper, Trait. pal. vég. vol. i. p. 265, and yol. 
lil. p. 453. 
1871. Hquisetum Lyelli, Schenk, Paleeontographica, vol. xix. p. 207, pl. xxii. 
figs. 10-18. 
1882. Hquisetites Lyelli, Renault, Cours bot. foss. vol. 1. p. 150. 
1889. Equisetum Lyelli, Fontaine, Potomac Flora, p. 65, pl. i. fig. 7; pl. 1. 
figs. 4 and 5. 
Type. Small pieces of stem of an average thickness of 1°5 em. ; 
internodes about 2°5 cm. in length, the leaf-sheaths divided into 
numerous linear acuminate teeth. The surface of the internodes 
shows faintly marked and numerous longitudinal lines. Single 
branches at the nodes of frequent occurrence. Sporangia and 
spores unknown. 
The majority of the specimens in the British Museum are pieces 
of stems showing the leaf-teeth projecting some distance above 
each node, and also in a few instances there is a thin brown film 
on the surface of the internodes. This brown surface layer is 
all that remains of the plant tissues, and probably is simply the 
remains of the epidermal layer of cells; it is well shown in 
specimens V. 59 and V. 2864. 
