220 W. H. BALL 



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. 



In some regions, as the west coast of the Floridian peninsula, the 

 strata may be slightly inclined so that the beds between which sub- 

 terranean waters move have their edges beneath the sea. Torrential 

 rains in the interior of the peninsula carry vegetable matter into the 

 interstices of the soft limestone rocks, where it decays with the accom- 

 panying production of carbon dioxide and sulphuretted hydrogen gas. 

 This accumulates and under the hydrostatic pressure of an excep- 

 tionally heavy rainfall is sometimes forced out beneath the sea from 

 the edges of the submerged strata in sufficient volume to kill by suffo- 



' "Genesis of the Tertiary Species of Planorbis at Steinheim," Anniv. Mem. Boston 

 Soc. Nat. History, 1880, pp. 114, pis. I-IX, 4to. 



* "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- 



