NO. 1256. FOSSIL SHELLS OF THE COLORADO DESERT— STEARNS. 283 
In revitnviiig the forms herein considered it will hardly l)e ques- 
tioned that the variation the}' exhibit is correlated with salinity, 
using" the word as synonymous with alkalinity, or the mineral character 
of the waters in which the shells are found. In connection with the 
foregoing- the suggestion arises that some of the many springs now 
obsolete contained what in common parlance may be called a distinct 
species, or shells that were characterized Iw a single facies of sculp- 
ture, etc. Thus, in Mr. Orcutt's Indian or Fish Springs specimens the 
surface is finely granulose. His Dos Palmos shells are spirally threaded, 
though these aspects of sculpture are not alwa^^s — that is to say. in all 
the specimens — equally conspicuous. The Saratoga Springs examples 
of the Death Valley expedition are, like those of Michoacan, Durango, 
and New Mexico, smooth surfaced. The few Sevier Lake (Utah) speci- 
mens were sculptured, but so nmch eroded as to obscure the character 
of the surface. Doubtless many living springs remain to be explored 
and some of these mux furnish special facies of this versatile form, 
perhaps shells with the longitudinal sculpture only. With the above 
facts and suggestions before us, a coming together of these various 
forms would, it may be assumed, lead to hybridization and the phe- 
nomenal variation of P. jrrotea be thus explained. But here we must 
remember the fact that in all of the shells from all the localities, what- 
ever may be the surface characters in the main, the apex and early 
whorls are universally smooth. 
In the several hundred specimens recently received from the Flow- 
ing Wells locality, where the bodies of water are of considerable size, 
all of the varieties are included except Nos. 8 and 11. 
From the suggestion of variation through h^^bridization the follow- 
ing hypothetical views as to the formation of sculpture may be 
indulged in: 
The spiral sculpture or Urate character may be due to puckering of 
the mantle upon its being withdrawn into the shell, which would cause 
an increased deposit at certain points along or upon the edge of the 
outer lip. the lirate and intermediate depressed grooving correspond- 
ing to the wrinkling of the mantle edge. In examples that show the 
thread-like ridges on some whorls and not on others we may suppose 
that the shell-forming material, or lymph, as it may be called for con- 
venience, was less abundant one time compared with another. 
Where the double .system of sculpture is exhibited the forming of 
the lirate ridges was at times interrupted by a short period of rest, 
when the rim or edge of the mouth received the greater proportion 
of the secretions, thus making the thread-like sculpture secondary to 
the longitudinal for the time being. It would seem that alternations 
in the volume of lymph deposited as suggested, or from pores around 
the edge of the mantle and possibh' somewhat molded or shaped by 
the foot, may furnish a hint as to the various facies of sculpture. 
