84 



mals. Very large stretches of bare sand have ahiiost no animals 

 except about the margin. 



THE BASIN ASSOCIATION 



The basin association occupies the circular or oval area in the 

 bottom of the blowout, from which the sand is being removed by 

 wind (PI. Ill, Fig. i). Plants which persist in the basin must there- 

 fore be able to withstand excavation. The only plants that can with- 

 stand undermining to any appreciable extent are the green milkweed, 

 Acerates viridiflora Ell., and its variety lanceolata (Ives) Gray, 

 which thrives even better than the type. These plants are sometimes 

 found in bunch-grass or in the Panicum pscudopitbcscens association, 

 but are much more numerous in the basins. The roots are very long, 

 and the plant is procumbent upon the sand. The blowout basins of 

 the Nebraska sand-hills are usually grown over with the grass Red- 

 ■fieldia. A few perennials persist in the basin if the sand-removal is 

 not too rapid. These are Lithospcrmum ginelini, Euphorbia corol- 

 lata, and Lcspedeza ca pit at a. 



Animals of the basin are principally invaders from neighboring 

 associations. The blowsand animals and interstitial species from 

 the bunch-grass make frequent incursions. At the margin of the 

 Cassia zone in a blowout at the Devil's Hole are seen many mutil- 

 lids, wasps, and spiders. The basin has several distinctive species. 

 Sand-wasps of the genus Tachytcs hover about the flowers of 

 Acerates; and Cicindela lepida and Stachyocneuiis apicalis, the color 

 of which matches that of the sand, are found all over the bare sand 

 of the basin. Lycosidae often burrow in the open sand. Terrapene 

 has been taken at the edge of a blowout, Cncmidophorus has been 

 taken in a burrow in a large basin, and one often finds the tracks of 

 mice and rabbits, and the sinuous trail of the blow-snake crossing 

 the basin. Mutillidae and their burrows are found in the bare sand, 

 and at the margin are Schistocerca alutacea and Mclanoplus Haz'idiis. 

 Other Cicindelidae than Cicindela lepida are quite frequently taken 

 in the basin. Alydiis, the peculiar ant-like bug, is represented by 

 several species. Like the plant life of the basin association, the ani- 

 mal life is very scanty, and represents the extreme conditions of the 

 combined effect of wind and sand. 



THE WINDWARD SLOPE ASSOCIATION 



The windward slope of the blowout is quite steep, and the sand 

 is continually sliding down by gravity. Usually the top of the slope 



