THE PALEOXTOLOGIC RE COED 597 



greatest bulk of invertebrates are restricted to the bottom of the shal- 

 low seas within the depth to which sunlight readily penetrates, that is, 

 a depth on the average not over 600 feet. The value of this observa- 

 tion to the paleogeographer and the student of fossil marine life lies 

 in the confirmation of paleontologists that continental seas are shallow 

 seas, to the bottom of which in most places sunlight permeates. These 

 seas are to be compared with the littoral regions of the present oceans, 

 and they are the areas that are most exposed to climatic and physical 

 changes, due to their proximity to the atmosphere and the lands. The 

 life of these waters is, therefore, subject to an environment that is 

 more or less changeable, and one of the basic causes underlying or- 

 ganic change. It is the invertebrates of the littoral and shallow seas 

 that the paleontologist studies. 



In the tropical and subtropical shallow seas one meets with the 

 greatest variety of life and with the brighter colored and more orna- 

 mental shelled animals, but we are much surprised when told that the 

 greatest number of individuals occur in the colder shallow waters of 

 the temperate and polar regions. Johnstone states, " There is little 

 doubt that the distribution of life in the sea is exactly opposite to that 

 on the land. The greatest fisheries are those of the temperate and 

 arctic seas. . . . Nowhere are sea birds so numerous as in polar 

 waters. The benthic fauna and flora are also most luxuriant." The 

 Bay of Naples has a "richly varied, but (in mass) a scanty fauna and 

 flora," and " at the very least the amount of life in polar seas is not 

 less than in the tropics." 2 



Marine life is also more prolific near river mouths of the tem- 

 perate zones, probably because of the great quantities of dissolved 

 " salts of nitrous and nitric acid and ammonia, and other substances 

 which are the ultimate food-stuffs of the plankton." Just outside of 

 the estuary of the Mersey in Lancashire there were "not less than 

 twenty, and not more than two hundred animals varying in size from an 

 amphipod (one fourth inch long) to a plaice (eight to ten inches long) 

 on every square meter of bottom " (Johnstone, 1909 : 149, 176, 195-6). 

 Finally the quantity of life in the shallow waters of the sea is not 

 directly governed by favorable habitat, such as shallow sunlight waters 

 in constant circulation and of equable temperature, but seems to be 

 primarily controlled by the amount of the minimal food elements. 

 Sea-water may be regarded as a dilute food-solution having the essen- 

 tial materials on which life is dependent. Of these nitrogen and the 

 compounds of silica and phosphoric acid are present in the smallest 

 amount. Johnstone tells us that " The density of the marine plants 

 will therefore fluctuate according to the proportions of these indispen- 

 sable food-stuffs" (234). "It is only the protophyta among the 



2 "Life in the Sea," 1908, 201-205. 



