EVOLUTION AND DISTRIBUTION OF TERTIARY FAUNAS 219 



the animals are almost exclusively carnivorous and largely subsist 

 on the abundant rain of dead organisms which slowly descends from 

 the surface layers of the sea. 



It has been customary to regard the loo-fathom line as constituting 

 a sort of boundary between the fauna of the shores and of the deeps. 

 This has a certain foundation in the fact that at greater depths no 

 living algae can exist for want of sunlight. A more or less constant 

 migration, casual or accidental, is constantly taking place between 

 the littoral region and the deeps, but it is so slow, and the process of 

 adaptation to the new conditions so gradual, that we may safely 

 regard the abyssal fauna as even geologically old. I have called 

 attention to certain features of the eastern Pacific and Antillean abyssal 

 faunas which illustrate these remarks in the introduction to a recent 

 monograph.' 



Freshwater and terrestrial invertebrates are subject not infre- 

 quently to one set of influences which is rarely noticed in the open 

 sea. This is, in the case of the limnophilous species, a change in 

 the mineral content of the water in which they live. This is usually 

 gradual and when injurious chiefly due to the concentration of salts 

 (which exist in all fresh waters arising from drainage) by evaporation. 

 In the case of many large Pleistocene lakes, of which the prehistoric 

 Lake Bonneville may be taken as an example, this process has been 

 carried on until the saline content of the water became so excessive 

 that all molluscan life became extinct, as in the Great Salt Lake of 

 Utah. A careful study of the beds of shell-marl deposited by the 

 shrinking lake shows that the effect of the gradually increasing 

 salinity of the water on the freshwater mollusks contained in it was 

 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 



1 Bull. Mus. Comp. Zoology, Vol. XLIII, No. 6, October, 1908, pp. 205-12. 



