Prairie- and Sand Hill Distriet. 541 
often filled with a dense growth of Zudwigia polycarpa, Ammannia coccinea and 
Penthorum sedoides while Eleocharis palustris forms thick carpets at the edges, 
associated occasionally with Nasturtium palustre, Mentha canadensis; Spiro- 
dela polyrhiza floats on the surface of the water. 
Wet Meadow Formation. This type of formation occurs more especially 
in the sand-hills and foothills, but occasionally in the prairies proper. — Rush 
Meadow Association: This type of wet meadow is frequent in the sub- 
sand-hills. The facies are Juncus tenuis, F. nodosus, Scirpus atrovirens and 
S. pungens (= 5. americanus). Juncus tenuis and Scirpus atrovirens are most 
commonly associated and with them Gl/yceria nervata. In more sandy meadows 
to the westward Juncus nodosus and 9. Torreyi are prevailing. — Fern 
Meadow Association: These meadows are confined to wet valleys and 
canyons of the Loup and Niobrara and the valleys of the principal prairie rivers. 
They consist of almost exclusively two facies Nephrodium (Aspidium) thelypteris 
and Onoclea sensibilis with which ferns are associated Zpilobtum lineare, Hype- 
ricum majus, Galium trifidum and Campanula aparinoides. — Sedge Meadow 
Association: This association is of rare occurence in Nebraska and then 
along the Missouri River and main streams. Carex stricta, C. filiformis var. 
lanuginosa, Spartina cynosuroides occur in such formations. 
Pond and Stream Formation. This exists in several well characterized types 
mentioned below viz.: 
Pondweed Association: This is the most widely distributed aquatic 
association of the prairie region. The facies are Potamogeton natans, P. amplı- 
folius, P. pauciflorus (= P. foliosus), Myriophyllum spicatum, Zannichellia 
palustris and Ceratophyllum demersum, while Potamogeton spirillus, P. hybridus 
(= P. diversifolius), P. zosteraefolius occuring as secondary species are rare. — 
Pond-lily Association: The ponds where aquatics grow are of two kinds; 
those in which the facies is Nuphar advena, and those in which Mymphaca 
tuberosa and Nelumbo lutea are found. The former type is found in small 
ponds in the sand-hills and especially in the sub-sand-hills. The second type 
occurs in’the edge of the wooded bluff country. Here the association of 
species is in large ponds and shows zonation of sedges and grasses. — Water 
Crowfoot Association: The facies of this type is Ranunculus circinatus 
(— Batrachium trichophyllum) with such species as Nasturtium offienale, Carda- 
mine hirsuta, Veronica americana and V. Anagallis associated with it. — 
Stonewort Association: Pools and small lakes, according to POUND and 
CLEMENTS, who have been followed in general in the description of the foregoing 
associations, possess a characteristic vegetation in which four species of 
Chara enter largely, viz: Chara contraria, C. foetida var. longibracteata, 
C. fragilis and C. coronata. The other plants present are Potamogeton and 
Naias flexilis. 
Algal Associations and Limnoplankton. Certain stagnant pools in the valley 
ofthe Middle Loup River, Nebraska, besides supporting Zemna minor, L. 
trisulca, Spirodela polyrhiza, Riccia fluitans, Utricularia vulgaris, U. minor, 
