1922] NATURAL SCIENCES OF PHILADELPHIA 71 



unbroken stretches of flattish sandy country may well be doubted, 

 but where, as here, the many mountain ranges form islands of a 

 slightly less degree of desiccation, the snails are given a chance to 

 maintain a figurative toehold, and in time to become better adapted 

 their desert environment, while meanwhile the principle of isola- 

 tion comes into play. It is only recently that it has been discovered 

 that if numbers of species are to be regarded as criteria, the true 

 molluscan metropolis of North America is not in tropic Florida, 

 nor in the swampy lowlands of Louisiana and Texas, nor in the 

 forests of the northwest, but in dry Arizona, where every mountain 

 range, in some cases almost every canyon, seems to harbor its 

 own peculiar land-snail fauna. There are of course numerous 

 widespread species, principally minute, but the multitude are 

 surprisingly local in their distribution. While it is perhaps not 

 to be expected that the region under discussion will prove to rival 

 southern Arizona, enough has now been done to show that further 

 exploration will yield substantial rewards. The land snails natur- 

 ally constitute the great unknown. Nevertheless fluviatile mollusks 

 likewise are more important and numerous than might be suspected. 



The Colorado Desert. — The term "Colorado Desert" has 

 been somewhat variously applied by different writers, but the most 

 logical as well as historically the most precise use of the name is 

 undoubtedly the definition given by its original proponent, Prof. 

 W. P. Blake, who writes (:14, p. 6-7; :15, p. 21-22):— 



'The name 'Colorado Desert' was given to this region by the 

 writer in 1853." 



"This was before the State of Colorado received its name. It 

 was deemed most appropriate to connect the name of the Colorado 

 River with the region inasmuch as the desert owes its origin to 

 the river by the deposition of alluvions and the displacement of 

 the sea-water." 



"A tendency is shown by some writers to extend the area 

 known as the Colorado Desert so as to include the arid regions 

 north of it, especially the mountainous region along the Colorado 

 ami the Mojave, partly known today as the 'Mojave Desert.' 

 This was not the intention or wish of the author of the name. It 

 was intended to apply it strictly to the typical desert area of the 

 lacustrine clays and alluvial deposits of the Colorado where extreme 

 characteristic desert conditions prevail, such as arid treeless plains, 

 old lake beds and sand hills — such conditions as are found in the 

 Sahara of Africa and in the delta regions of the Nile. I should also 



