282 PROCEEDINGS OF THE NATIONAL MUSEUM. voi..xxiv. 
distribution 80 much farther in that direction than before known as to 
warrant the expectation of the tinding- of /*. proUa in some of its 
man}^ varieties at intermediate localities. 
The eighteen examples figured as representatives of the many facies 
of this exceedingly variable form are connected in one character or 
another by innumerable individuals which blend or intergrade, and 
which it would be impracticable to describe. The above are numbered 
in the IT. S. National Museum Register 47854, with second or index 
numbers 1 to 18, in as many tubes corresponding to the number of 
each figure as given in this paper. 
With the exception of the New Mexico and Durango localities, all 
of the others are represented in the U. S. National Museum collection. 
The occurrence in the Colorado desert of California, of the well- 
marked form described b}- the late Dr. Stimpson as Tryon'm cJofhrata 
has not been verified by any of the collections made b}^ various parties, 
as well as uwself ; not a single example having been detected in the 
thousands of specimens examined. The only localit}^ where it is defi- 
nitely known to occur is in the Pahranagat Valley, Nevada, where 
Dr. C. Hart Merriam collected a number of living specimens in a hot 
spring. The dead bleached shells collected forty years ago b}- Prof. 
W. P. Blake, and later by General Carlton in 1861-62, were probably 
found somewhere in the region of the Merriam locality, which is at a 
high altitude, 3,000 feet or more, in southern Nevada. There is a 
vast area between the Pahranagat and California localities practically 
unexplored; of its molluscan life scarcely anything is known. 
As to the causes of the extreme variation exhibited by the shells of 
this species, I will repeat what I have heretofore written by quoting" 
from the '"'Report on the Land and Fresh- Water Shells collected in 
California and Nevada by the Death Valley expedition,""^ as follows: 
The suggestion that arises from the study of the forms above reviewed and the 
regions and conditions to which they are related point to the causes that induce 
variation and to the permanency of species and genera, or to the mutabihty of the 
same, as dependent on environmental characteristics. 
If we are warranted in assuming * * * that Avith a volume of 
water ample or maximum and chemical proportions as related to vol- 
ume minimum our Tryonkoi would be smooth, and that the smooth or 
sculptureless surface that so generally prevails in the Bytli'inelhiH and 
related groups is, in a conventional sense, normal, then we may rea- 
sonablv assume that to the opposite of these conditions, with volume 
of water variable or minimum and chemical proportions as related to 
volume of water increased or maximum, the phenomena of variation 
may be attri?>uted. That fluctuations in volume of water in the springs, 
pools, lagoons, etc. , throughout the entire desert region above descrilied 
are occa.donal, if not of frequent occurrence, is well known, and in 
some years the maximum is extreme, as has been pointed out. 
^ North American Fauna, p. 280, U. S. Department of Agriculture, 1893. 
