to desert areas where the paucity of favorable molluscan habitats makes the 

 casual, hit and miss type of search often result in total failure, whereas the 

 same technique in more humid regions usually results in a representative 

 sample of at least the more common forms. 



Desert collecting involves no paraphernalia or techniques not used in 

 other regions. Since these matters have been covered eLsewhere in this 

 booklet, the remarks below are directed to the two points mentioned above. 



The Desert Environment. The major area of North American desert is 

 located in the western part of the continent, extending from southeastern 

 Oregon and southern Idaho southward into Mexico as far as the Yaqui 

 River, including most of the peninsula of Lower California. The continuity 

 of this desert region is interrupted, of course, by the highlands of Utah, 

 Arizona, and New Mexico. Within the broad area outlined above 4 distinct 

 desert regions are recognized, the Great Basin, Mojave, Sonoran, and Chi- 

 huahan. Each of these desert regions differs from the others in details of 

 climate, physiography, flora, and fauna. This paper was written with the 

 Great Basin desert in mind, but it may apply, at least in a general way, 

 to other areas as well. 



The chief characteristics of a desert climate are: high temperatures 

 with extreme daily and monthly ranges; low, infrequently distributed rain- 

 fall; low humidity; high evaporation; and a high percentage of cloudless 

 days with high light intensity. 



Physiographically the Great Basin consists of isolated, roughly parallel 

 mountain ranges separated by nearly level basins or intermountain plains. 

 Some undrained basins contain playa, or alkali, flats, with fine-textured 

 soils, most of them with high concentrations of salts. The lower slopes of 

 the mountains are covered by alluvial fans of coarse to fine materials. 



A characteristic plant association has become adapted to this general 

 environmental situation. It exhibits certain common features, notably in 

 the adaptation of roots, stems, and leaves to withstand drought; the wide 

 spacing of individual plants; slow growth; adaptation of life cycles to periods 

 of abundant moisture; adaptations to high salt concentrations in some 

 situations; and adaptations to the textural variations of the soil. 



Ecological Requirements of Mollusks. The absence of a given kind of 

 organism from a region does not necessarily imply that it could not exist 

 there. Its absence may be due to insufficient means of dispersal, barriers 

 that prevent it from reaching the area, or merely insufficient time to do so. 

 Consideration of these topics would involve delving into problems beyond 

 the scope of this paper. 



In general, the mo^t important problem for mollusks is desiccation, 

 stemming from the fact that their soft bodies, devoid of impervious outer 

 covering, must be kept moist at all times. Other things being equal, per- 

 manent bodies of water provide the most favorable habitats. Most mollusks 

 are aquatic and have solved the moisture problem, but for terrestrial forms 



(73) 



