78 



POST-TERTIARY ENTOMOSTRACA. 



Some of the beds must belong to the earlier, and some to the more recent portions of 

 the epoch. 



In considering the effects of fluctuations of climate upon marine animals, we 

 must also take into account other modifying causes, such as depth of water, character of 

 the sea-bottom, supply of food, and the abundance or otherwise of the enemies of 

 particular species. 



Without attempting to decide upon the special ages of the various Arctic deposits 

 examined for the purposes of this paper, we must have made evident the fact that many 

 differences exist between them. 



1. The varying position of the Arctic shell-clays, with respect to the Boulder Clay, 

 has been described. 



1. Some Arctic shell-clays are covered by masses of highly glaciated Boulder Clay. 



2. Other Arctic shell-clays are intercalated between masses of Boulder Clay. 



3. Other Arctic sliell-clays rest upon the Boulder Clay^ and not only are not covered 

 by it but are followed by a regular succession of beds, showing a very gradual change 

 from old marine to recent estuarine conditions, and entirely free from any indications of 

 violent breaks, 



a. Beds of this class are deposited in hollows of the Boulder Clay, and have thus been 

 formed subsequently to an extensive denudation of that material. 



h. They contain large boulders, not derived from the Boulder Clay on which they 

 rest, but which have been dropped upon existing sea-bottoms full of marine life. 



c. They enable us to trace the coming and going of the Arctic fauna by the 



examination of the clays at the lower and upper parts of the same section, as was 

 indicated in an account of a Paisley pit. 



d. They are often immediately succeeded by a Cardium edule bed of a modern period, 



without any intervening Boulder Clay at all. 



e. Sometimes the passage from marine through estuariue to freshwater beds can be 



followed by examining samples of one bed of clay taken from different points in 

 the section. 



An example of this was given from the tileworks near Girvan, Ayrshire. 



II. Some Boulder Clays have themselves been distinguished as fossiliferous aad as 

 being Arctic shell-bearing clays, although found under special circumstances. 



III. The study of the special fauna of each bed reveals also distinctions between 

 them. 



1. The shell-beds represent differences of depth, some being littoral and others 

 deep sea. 



