BUFFALO SOCIETY OF NATURAL SCIENCES 73 



sea as to have affected the fauna very markedly, but it will be noted 

 that marine types must predominate in the fauna of the Baltic, as 

 a whole, since for none of the phyla are the numbers much greater 

 than they are at Kiel, where the fauna is wholly marine. 



Perhaps the most significant fact brought out, is that the marine 

 forms which are found in the Baltic, though they may be dwarfed 

 or otherwise modified, are not different specifically from the marine 

 forms found along the coasts of Great Britain, nor do the fresh-water 

 forms differ from those found in the rivers emptying into the Baltic, 

 or those in the neighboring fresh-water bodies. Thus it is established 

 that in a brackish-water body of the nature of the Baltic the fauna is 

 due to the mingling of modified marine and of modified fresh-water, 

 that is, river forms. Only the more euryhaline marine species sur- 

 vive and these may in a given estuary give rise to a fauna which 

 we may designate as a " brackish- water fauna." It will consist of 

 forms derived in the manner just described, and these forms may be- 

 come adapted to the peculiar temperature and salinity conditions 

 prevailing in the given estuary. Thus, new mutations, varieties, and 

 occasionally species may arise, but seldom a new genus and never a 

 whole class of organisms. It is only the smaller taxonomic divisions 

 which are affected. Furthermore, a "brackish- water fauna" in any 

 estuary is always ephemeral, for the estuary is of short duration, 

 geologically speaking. 



The Severn Estuary. While the Baltic serves to show on a large 

 scale what happens to a marine fauna which is gradually subjected 

 to fresher and fresher water until it passes through brackish condi- 

 tions to entirely fresh ones, there is another type of brackish water, 

 the estuary, which is often said to have a fauna of its own. An estu- 

 ary may be defined as the drowned lower portion of a river in which 

 twice daily there is a change in the water from fresh to marine and 

 back again as the tide comes in and goes out. On account of the 

 tidal scour and thorough mixing of the marine and the inflowing river 

 water, the brackish portion will not be very large. The Severn, on 

 the west coast of England, is a very typical estuary, having the long, 

 slowly broadening form toward the sea. There are a number of trib- 

 utaries with their respective estuaries, so that on the whole the 

 Severn may be considered characteristic. It is well known that muds 

 are the dominant sediments, not only in the main tidal channel far 

 out to sea, but also in all of the tributary channels. Professor W. J. 

 Sollas has made a careful study of these muds in order to determine 



