EVOLUTION AND DISTRIBUTION OF TERTIARY FAUNAS “sol 
to lead to a thickening and corrugation of the shell, a tendency to 
longitudinal ribbing, and a diminution in average size, all of which 
changes may perhaps be due directly to the astringent action of the 
salts of sodium and magnesium upon the thin and delicate margin 
of the mantle which secretes the additions to the shell. ‘These char- 
acteristics become more and more pronounced as the waters become 
more saline, until finally the conditions become too rigorous for 
survival. The gradually intensified effect of the increase of salinity 
may be beautifully illustrated by a collection of the fossil shells from 
the successive marl beds around Great Salt Lake. Another instance, 
probably of the same nature, is afforded by the marls of Steinheim, 
in Wurtemburg, of which the mutations shown by the species of 
Planorbis, in particular, are described in the well-known monograph 
by Hyatt.’ 
A somewhat similar effect seems to be produced in the case of 
landshells inhabiting arid volcanic islands in windy regions. Here 
the astringent effect appears to be produced by the alkaline volcanic 
dust to which these animals living on almost bare shrubs or among 
sparse herbage are more or less constantly exposed. I have called 
attention to the conditions under which this effect seems to be pro- 
duced in a paper on the landshell fauna of the Galapagos Islands.? 
This illustrates how upon animals of quite different systematic rela- 
tions, similar effects, simulating an apparent convergence, may be 
caused by the direct action of the environment upon individuals. 
Paleontologically these instances are worth noting, as otherwise the 
forms concerned might well be regarded as belonging to totally dif- 
ferent groups from the individuals which developed normally in an 
ordinary habitat. 
In conclusion I may call attention to certain factors which have 
- serious importance in modifying the fauna of a large extent of coast 
catastrophically, and which inferentially are to some extent responsible 
for the marked changes we observe in different stratigraphic horizons 
where we do not find indications of coincident orogenic changes. 
t “Genesis of the Tertiary Species of Planorbis at Steinheim,” Anniv. Mem. Boston 
Soc. Nat. History, 1880, pp. 114, pls. I-IX, 4to. 
2 “Insular Landshell Faunas, Especially as Illustrated by the collection of Dr. G. 
Baur on the Galapagos Islands,” Proc. Acad. Nat. Sciences, Philadelphia, August, 1896, 
PP- 395-459: 
