288 PROCEEDINGS OF THE NATIONAL MUSEUM. vni.xxiv. 
the maximum of development in this direction is farther to the south, 
as shown by the fine, large, highly polished shells of the west Mexi- 
can species P. (Aplexa) aurmitia Carpenter, of Mazatlan. This form, 
which the author remarks as "not common" at that place, sometimes 
measures nearly an inch and a half in length.^ As Llmncm i<tag nails 
and L. megasoma of more northerly latitudes exhibit the culmination in 
size of the LmiiKBldce^ so in the south does P. aurantia of the l*1iy)<!(l<E. 
That several species of Limncea occur at numerous localities in the 
Great Basin to the north, at various elevations from 1,300 to 4,0()0 feet^ 
and upward, and eastward in Arizona at Tucson, elevation 2,300 feet, 
to the higher altitude of Walker Lake, in the San Francisco Moun- 
tains, 8,250 feet, and (Z. hidhnoidei<) still nearer, both in distance and 
altitude, at Daggett, on the Mojave River, in the Mojave Desert, 2,000 
feet, points to the thermal factor of the lower levels in this region as 
the uncongenial feature which excludes Limncea from the desert. 
Thus L. Jiuinllts becomes a mountain species in the Sierra Laguna of 
Lower California, and other species of the genus occur to the south- 
ward not far from the coast, at lower levels, within reach of the cool 
winds and fogs of the Pacific. Evidence is not wanting to show that 
depauperation in the Limncea is coincident with high thermal condi- 
tions,^ where such temperatures are continuous or prevail the greater 
part of the year. 
In considering the L^hysas of Indio and other desert localities, a 
glance at Plate V shows many forms that are familiar to students of 
North American Lhnnojjhila. To facilitate comparison, on Plate IV 
are presented several figures from Binney * of described species from 
regions both north and south of the desert. With the distributive 
agencies previously indicated in mind, it ma}' be well to turn to these 
forms and note the localities at which they were detected. Call credits 
P. (jyrlna, which he suggests is a variety of P. heterostropJia, to the 
Upper Bonneville beds, near Salt Spring Creek, and the variety 
P. elliptica, to Warm Spring Lake, near Salt Lake City, both places 
in Utah. Of the latter he says it occurs "abundantly and of large 
size." The same author remarks of jP. heterostropha that it has not 
been found in the lake beds of either Bonneville or Lahontan, l)ut is 
abundant as a semifossil on the surface of Sevier Desert. Both 
P. lidenmtroplia and P. gyrlna were found living by the Death Valley 
expedition at many places in the Great Basin much farther south than 
the Lahontan and Bonneville regions. P. heterostroplM was collected 
^Over a hundred were obtained by me at Aeapulco in 1868. [R. E. C. 8.] 
^ Mr. Vernon Bailey collected L. palustris in the Uintah 3Iountains in a creek at an 
elevation of 10,000 feet. 
^Doubtless great extremes of temperature in either direction are detrimental to 
the existence of these forms. 
* Land and Fresh Water Shells of North America, Smithsonian ^Miscellaneous Col- 
' lections, No. 143, Part II, 1865. 
