46 THE OCEAN RIVER 



of marine sediments, two great classes of molluscs, the gastro- 

 pods or conchs and later the bivalves or clams, made their 

 entrance in turn on the floor of the continental seas, and have 

 persisted in varying abundance to the present day. Compared 

 to other groups of sea creatures these clams and conchs have 

 changed remarkably little over the three to four hundred mil- 

 lion years since they first appeared. Had human beings existed 

 during Jurassic time, about 120 million years ago, they would 

 have found oysters growing in the shallow seas which were 

 then intruded upon the continents. Though distinctly different 

 from present-day oysters, they were unquestionably oysters, 

 and large ones at that. The fishlike reptiles and ancient squids 

 and belemnites of that far period were not limited to a diet 

 of oysters, however, but could well have feasted upon the first 

 crabs and lobsters, probably descended from the by then 

 extinct trilobites, as they swam in seas where midwestern 

 farmlands now exist and in the ancient Sea of Tethys where 

 parts of which is now Europe stand. 



Some of the early groups of sea creatures, more persistent 

 than others, have repeatedly tried to dominate the marine 

 communities with new changes in their basic pattern. Each of 

 these variations in turn enjoyed its period of success and swam 

 in great numbers through those ancient seas. But each in turn 

 finally failed to meet the test of adaptation to changing con- 

 ditions and was replaced by a new outgrowth of the parent 

 stock. More active than the clams, oysters, and conchs are 

 those other molluscs, the cephalopods, which include the 

 squids, cuttlefish, and other relatives of the octopuses. Perhaps 

 because of their more dynamic way of life they underwent 

 greater changes as one experiment in evolution after another 

 failed to survive. The first to appear were housed in long, 

 straight, slightly tapering tubes or shells, gas-filled for buoyancy. 

 They sometimes reached a length of 12 feet or more. The 

 long shells were a considerable handicap for an active animab 



