245 
mm. long and 1% to 134 mm. wide. Carpels two-keeled and 
usually a little scabrate on the back. 
Ontario, New York, south to Florida, New Mexico and Mex- 
ico, west to Ohio and Minnesota. 
8. M. hippuroides. Nutt. in Torrey and Gray FI. 1, 530 (1840). 
M. scabratum. Cham. Linnza 4, 506, not Michaux. 
Leaves in whorls of fours and sixes, sometimes scattered, the 
floral 8 to 12 mm. long, linear-lanceolate, serrate or dentate, up- 
permost nearly or quite entire, lowest pinnatifid ; submerged, with 
six to eight pairs of capillary pinne. Monoecious, but occasion- 
ally with only one kind of flowers on the stems, and so appearing 
dicecious. Petals often pink-colored, sub-persistent. Stamens 
four. Fruit about 2 mm. long by 1 mm. or a little more in 
breadth ; carpels keeled and somewhat rough, with a deep groove 
between them. A western form nearly allied to our Atlantic JZ. 
heterophyllum. Coll. by Nuttall and Hall in the Wahlamet ponds, 
and by Howell at Sauvie’s Island, Oregon, and by Chamisso near 
San Francisco, E. L. Greene, Stockton, and A. B. Simonds at 
Clear Lake, California. 
9. M1. pinnatum. (Walt.) 
Potamogeton pinnatum. Walt. Fl. Car. 90 (1788). 
M. scabratum. Michx. Fl. 2, 190 (1803), and most Amer- 
ican authors. 
Leaves in whorls of threes and fives, sometimes scattered, the 
floral linear, pectinate, toothed or cut-serrate, the teeth compara- 
tively few, 5 to 15 mm. long, gradually changing into the sub- 
merged, which are in crowded verticils, the capillary pinnz sparse. 
Spikes 10 to 20 cm. long. Petals purplish, somewhat per- 
sistent. Stamens four, very rarely six. Mature fruit about 13; 
mm. long, and 114 mm. broad. Carpels strongly two-keeled and 
scabrate on the back, the grooves deep. 
Shallow water, Rhode Island to Florida, Louisiana, Texasand 
Missouri. 
10. M. Metuisin S. Watson Proc. Am. Ac. 25, 148 (1890). 
Stems stout, much branched. Leaves in verticils, or subver- 
ticils of fours and fives, or scattered, the floral pinnatifid or toothed, 
‘the divisions remote, 10 to 15 mm. in length; submersed pin- 
natifid, 20 to 30 mm. long, the divisions finely capillary, 
