268 PTERIDOPHYTA. [CH. 



The Keuper species E. arenaceus is, however, more com- 

 pletely known. The specimens referred to this species are 

 very striking fossils; they agree in all external characters 

 with recent Horse-tails but greatly exceed them in dimensions. 



4. Equisetites arenaceus Bronn. 



This plant has been found in the Triassic rocks of various 

 parts of Germany and France ; it occurs in the Lettenkohl 

 group (Lower Keuper), as well as in the Middle Keuper of 

 Stuttgart and elsewhere. The species may be defined as 

 follows : — 



Rhizome from 8 — 14 cm. in diameter, with short internodes, 

 bearing lateral ovate tubers. Aerial shoots from 4 — 12 cm. in 

 diameter, bearing whorls of branches, and leaf-sheaths made up 

 of 110 — 120 coherent uni-nerved linear segments terminating in 

 an apical lanceolate tooth. Strobili oval, consisting of crowded 

 sporangiophores with pentagonal and hexagonal peltate termi- 

 nations. 



The casts of branches, rhizomes, tubers, buds' and cones 

 enable us to form a fairly exact estimate of the size and 

 general appearance of this largest fossil Horse-tail. The 

 Strassburg Museum contains many good examples of this 

 species, and a few specimens may be seen in the British 

 Museum. In the Ecole des Mines, Paris, there are some 

 exceptionally clear impressions of cones of this species from a 

 lignite mine in the Vosges. 



It is estimated that the plant reached a height of 8 to 

 10 meters, about equal to that of the tallest recent species of 

 Equisetum, but in the diameter of the stems the Triassic plant 

 far exceeded any existing species. 



It is interesting to determine as far as possible, in the 

 absence of petrified specimens, if this Keuper species increased 

 in girth by means of a cambium. There are occasionally found 

 sandstone casts of the pith-cavity which present an appearance 

 very similar to that of Calamitean medullary casts \ The 



1 Jager (27). 



