298 
23. Cosmocladium tumidun n. sp. XX 750. 
24. Cosmarium pseudoprotuberans Kirch. X< 750. 
25. C. Pacificum n. sp. 750. 
26. C. Gotlandicum Wittr. X 750. 
27. C. elegantissimum Lund. > 750. 
28. C. pericymatium Nordst. X 750. 
29. C. synostegos Schaarschm. X 750. 
30. C. porrectum Nordst. 560. 
31. C. costatum Nordst. X 750. 
32. C. margaritatum (Lund.) Roy and Bissett. 560. 
33. C. subspectosum var. validius Nordst. < 750. 
34. Stavzrastrum leve Ralfs. % 750. 
35. S. protractum nov. nom. 560. 
The Genus Cenchrus in North America. 
By Gero. V. NASH, 
Cenchrus is a small genus of grasses containing about a dozen 
species, found principally in tropical and sub-tropical regions. 
In the United States it extends into the temperate zone. One 
species, C. tribuloides, the common bur-grass, occurs frequently in 
sandy soil along river banks, and has a very wide distribution, €x- 
tending from Massachusetts to Ontario, Minnesota and Nebraska, 
south to Mexico. It is also found in South America and in Cuba. 
The other species occurring in the United States are confined to 
the southern parts of the country, one, C. echinatus, extending as 
far north as North Carolina. 
The genus in the United States is readily divided into two 
well-marked groups, the first containing those forms in which the 
involucre consists of two spine-bearing valves, more or less con- 
cealing and enclosing the 2-6 spikelets, as illustrated in the com- 
mon C. tribuloides ; the second has only one species, the involucre 
consisting of 2-4 rows of erect bristles, the outer shorter than the 
inner, the single spikelet merely surrounded by the bristles and 
not enclosed. The single species belonging to this group is C. 
myosuroides, which ranges from Georgia and Florida to Texas, 
and south through Mexico to South America, where it is very 
common. 
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