No. 499] 



475 



Atlantic. 4 De Lappa rent-' states that this western por- 

 tion has a few of the Atlantic species but all of reduced 

 size. A comparison of British and Spanish coast species 

 gives the same result. Haliotis tuberculatum is larger 

 at Guernsey than on the Spanish coast. The difference 

 of temperature between the two localities may be another 

 factor in causing - this dwarfing'. 



(c) Change clue to an Increase of H 2 S. — The presence 

 of much of this heavy gas in an enclosed or partially en- 

 closed basin would prevent the presence of living organ- 

 isms and hence the only fauna which sediments deposited 

 here could contain, would he free-swimming or floating 

 individuals. This pelagic fauna contains besides fish, 

 pteropods, and especially larval forms of almost every 

 animal group. Thus the sediment of such an enclosed 

 basin would contain small shells, embryonic in character, 

 pteropods and a few fish. Andrussow 7 has shown that 

 in consequence of the greater salinity and density of the 

 deep water, the Black Sea shows only slight evidence of 

 vertical currents. Such currents are apparent only to a 

 depth of 125 fathoms, and hence only to this depth is there 

 sufficient oxygen for the support of animal life. At a 

 depth of 100 fathoms the separation of H 2 S is observable, 

 increasing in amount with the increase in depth. The 

 separation of H 2 S is regarded as due to the agency of 

 microbes (Sulfobacteria) living upon animal remains of 

 the free-swimming and floating forms of life sunk to the 

 bottom. It is attributable in part also to the derivation 

 from sulfates. Hand in hand with the separation and 

 enrichment in H 2 S is the diminution in sulfates in the 

 sea water, the separation of the carbonates and of FeS. 

 In the great depths of this sea the bottom is covered with 



free-floating diatoms, fragments of quite young pelecy- 

 pods, and minute grains of CaC0 8 , and much FeS. 



