THE FERX ALLIES. 223 
keeled, concave on the inner side, the intermediate nearly 
three times smaller, subulate lanceolate; those of the fertile 
branches nearly equal; spike sessile terete. On mountains. 
ISOETES, Linnaeus. 
GKS. CHAR. Fructification enclosed -within the 
swollen base of the leaves. 
1. I. lacustris, Linnaeus Quillwort. Rhizome a 
blunt tuber; leaves subulate, roundish- quadrangular, 
with four longitudinal jointed tubes, broad and flat at 
the base. In lakes and ponds. 
FILULARIA, Linnceui. 
GEN. CHAR. Fructification consisting of coriaceous 
four-celled spore-cases, seated on the rhizome. 
1. P. globulifera, Linnaeus. Pillwort. Rhizome 
slender creeping leafy at intervals ; leaves very slender 
erect; spore-cases axillary nearly spherical, hairy. On 
the margin of ponds and lakes. 
EQUISETUM, Linnaws. 
GEN. CHAR Fructification forming terminal cones, 
the spore-cases attached to peltate scales. 
Fertile and barren stems dissimilar. 
1. E. Telmateia, Ehrhart Barren stems smooth 
with 30 ridges, branched: sheaths of branches with 
subulate two-ribbed teeth ; fertile simple short, with large 
crowded 30-40 toothed sheaths In muddy places. 
2. E. umbrosum, Willdenow. Barren stems 
rough, with 20 ridges, branched: sheaths of branches 
with subulate one-ribbed teeth ; fertile simple short, with 
Crowded pallid 12-20 toothed sheaths. In damp glens.j 
3. E. arvense, Linnaus. Barren stems roughish 
v.-ith 10-16 ridges, branched: sheaths of branches with 
