30 THE CHICAGO ACADEMY OF SCIENCES. 
the series, molluscan life disappears altogether. The evaporation of 
the water caused an increase in its alkalinity, and this condition of the 
environment is reflected in the shell, which becomes more solid, smaller, 
and ribbed or corrugated.* 
Some years ago Dr. W. H. Dall? penned the following lines, which 
admirably describe the effect of unfavorable environments on fresh- 
water shells: 
“The extra development of plicate sculpture is generally associated 
in arid regions with the dryness, and in moist regions with the presence 
of some alkaline salt, which accentuates the action of those factors in 
the organism which are concerned in the formation of the minor irregu- 
larities of the shell surface. The manner in which this is brought about 
is one of the prettiest illustrations of the direct action of the environ- 
ment which I know, and seems to be sufficiently established by both 
geological and physiological evidence. 
“In the arid region of the far West, especially in the desiccated 
lake basins of Utah, Nevada and California, it has long been observed 
by the writer, Dr. R. E. C. Stearns and others, that in the successive 
beds of fresh-water marl, which the now dried-up lakes deposited in 
Pliocene and Pleistocene times, the shells indicate a progressive change 
in surface characters as the alkalinity of the water increased, until at last 
the amount of alkali became so great that the mollusks were extermi- 
nated or found a precarious refuge in the fresh-water streams which 
fell into the basins in question. The shells, without regard to genus 
or systematic relations, showed a unanimous tendency to become ridged, 
plicated or rugose; the regularity of the gastropod coil was interfered 
with, abnormalities became more common, and, toward the last, almost 
general. Projecting sculpture, spiral threading, carinz, riblets, etc., 
were exaggerated; size generally diminished, the height of the spire 
relatively to the diameter became less, and general degeneration curi- 
ously combined with extreme accentuation and irregularity of surface 
characters. Something of the same sort is visible at the present time 
in the shells of fresh-water gastropods in the irrigating ditches of farms 
in the alkaline arid region; those shells, in the ditches where the 
water has leached out alkaline matter from the soil, showing evidences 
of change in the same direction in surface sculpture, as I have per- 
sonally observed in the Honey Lake Valley, Nevada. 
“The dynamical origin of these changes may be explained by con- 
sidering the origin of the surface characters of the shell. The de- 
1See Call. Bull. U. S. Geol. Surv., no. 11; Stearns, Proc. U. §. Nat. Mus., 
XIV Dp. 221. 
?Proc. Phil. Acad., 1896, pp. 407-409. 
