Life in the Ancient Seas 309 



Idea of what types successively dominated the waters 

 of our globe. It should be borne in mind, however, 

 that all reliable evidence we have of ancient animal life 

 is in the nature of skeletons or other hard parts which 

 have been retained more or less intact or have left their 

 imprint in the rocks. Undoubtedly, there have existed 

 numerous forms whose bodies were so soft throughout 

 that they left no trace whatever. And doubtless, too, 

 many hard creatures lived whose parts have not been 

 preserved or which have not yet been found. Indeed, 

 when we come to study the land surfaces and the sea 

 floors of the present time, it becomes quite obvious that 

 there are so few favorable conditions for the preser- 

 vation of the remains of either terrestrial or marine 

 life that if former geological conditions were the same, 

 which is not unlikely, the occurrence of fossiliferous 

 remains may be looked upon as the result of fortunate 

 accidents. 



In a preceding chapter it is mentioned that the trilo- 

 bites are among the oldest known fossils. But it is 

 quite certain that before they fairly had a good start 

 the cephalopods appeared upon the scene to dispute 

 their sovereignty. Among these, the nautilus and the 

 ammonite seem to hold a prominent place. Our pres- 

 ent living nautilus is the sole survivor of what in those 

 early times was a very numerous race. More than 

 2,500 species once swam the Silurian seas, a time in the 

 history of our continent when the waters of the Pacific 

 covered the western half of New York State. The 

 oldest forms have straight shells; but in the course of 

 time coiled shells became conspicuous; later, the race 

 declined, and then they are found sometimes uncoiled 



