64 THE POTOMAC OK YOUNGER MESOZOIO FLOllA. 



closely appressed, 5™" to 10'°'" long; teeth visible on the upper surface of 

 stems and ])ranches, 5 to 6. They are usually linear, lanceolate, acute, 

 rarely somewhat obtuse, and approximate. 



Locality: Fishing hut above Dutch Gap Canal. 



This pretty little Equisetum has not been found anywhere except at 

 the single locality given above. At this place it is rare. This species in 

 general appearance much more nearly approaches living forms than E. 

 Bogersi, which is its last predecessor in this region. It is very close to 

 E. Burcliardti Schimper, from the Wealden of Grermany, and perhaps 

 should 1)6 identified with that species. It does not, however, show the 

 swollen sheaths and the wide divergence of the teeth seen in the Wealden 

 si^ecies. Still, something of both of these features may sometimes be seen. 



The most noteworthy features of the Potomac species are the copious 

 branching and tlie slight diminution in the size of the successively formed 

 branches. In many cases there is little or no difference in the size of the 

 stem and the branch which it sends off. Tlie mode of branching seems 

 to be quite often dichotomous, but this may possibly be the effect of 

 compression. The specimens are often partially or wlioUy decorticated 

 so as to show no teeth, and in most of the specimens the teeth appear, 

 from maceration, to be narrower and fixrther apart than they were 

 originally. 



The unusual forms shown in PI. II, Figs. 1-3, appear to be portions 

 of the main stem, or at least of the primary branches They show six 

 teeth, while the smaller and much more conmion forms show five. The 

 enlarged portion, PI. 1, Fig. l", gives what seems to be the normal original 

 form of the teeth. They are closely approximate, and have a short acute 

 tip. PI. I, Fig. C, shows a slightly different form of. teeth. They are 

 blunter and farther apart at the tips. This is perhaps due to distortion 

 from pressure and partial maceration. 



Tliis plant must have attained a considerable heiglit. It has much 

 resemblance to the genus Casnarii/a in the mode of branching, its copious- 

 ness, the great length of the branches, and their woody character. 



