308 Systematic Paleontology 



While the reference of this form to Selaginella is not conclusive it is 

 certainly very suggestive of that genus in its superficial appearance, and 

 no better disposition of it has suggested itself. Possibly future collections 

 will fortunately disclose fruiting specimens vs^hich will conclusively settle 

 the question. ITeer ' has described a doubtful species from the Ceno- 

 manian (Atane beds) of Greenland, and Velenovsky ^ another which is 

 very similar to the Maryland form from the Perucer schichten of 

 Bohemia. 



Occurrence. — Patapsco Pokmation. Vinegar Hill, Maryland. 



Collection. — Johns Hopkins University. 



Order EQUISETALES 



Family EQUISETACEAE 



Genus EQLJISETUM Linne 

 [Sp. PI., 1753, p. 106] 



Perennial rush-like plants with jointed stems, which are generally 

 ridged and grooved, the ridges alternating at the joints. Much branched 

 in the Potomac forms, the recent species often simple. Eoot stocks sub- 

 terranean, often tuber-bearing. Leaves reduced to sheaths at the joints, 

 the sheaths toothed. Spores (as far as known) all of one size and shape 

 (isosporous). Sporangia borne on modified sporophylls (sporangio- 

 phores) which are aggregated to form a definite cone (strobilus). 



These plants, which in the allied order Calamariales forms so dom- 

 inant and imposing a group in the Paleozoic is almost entirely repre- 

 sented in Cretaceous formations by forms referable to the family 

 Equisetacese. A number of Carboniferous species of small size, which are 

 based upon impressions of cones or stems with leaf sheaths like those 

 of the modern forms, have been referred to the genus Equisetum (or 



^Selaginella arctica Heer, Fl. Foss. Arct., Band vi, Ab. ii, 1882, p. 39, pi. 

 xiii, fig. 5. 



^ Selaginella dichotoma Velen., Farne bohm. Kreidef., 1888, p. 29, pi. ii, 

 figs. 8-11. 



