860 TEXTBOOK OF ZOOLOGY 



ancient seas and their life. If we examine the life in a modern 

 tropical sea as shown in a Williamson motion picture, it is difficult 

 to realize how few of the objects we are viewing are apt to leave a 

 permanent record. 



The corals, of course, will survive eternally. The sea anemones 

 will leave no trace unless some individual may die and leave the 

 impression of its body in the mud before the tissues are disinte- 

 grated by bacteria. Most of the gorgeous fishes will leave no rec- 

 ord. As one grows ill and weak it is attacked by its fellows and 

 the leftover scraps if they reach bottom will be snatched up by 

 Crustacea. Only an occasional tooth or vertebra Avill survive. 



Sometimes in geologic history a great underwater volcanic deto- 

 nation would kill all of the fish in a small area of the sea, laying 

 them down to be buried in the mud. Thus we get aji occasional 

 glimpse of masses of tangled fish skeletons furnishing us a clue to 

 the great numbers of fish which have lived and of which we have 

 no record. Volcanic detonations of this sort were more common in 

 Devonian time than in ajiy period before or since. For a long time 

 geologists incorrectly called the Devonian "The Age of Fishes," 

 although we know now that since this time, there have been many 

 more kinds of fishes and more of a kind. 



Returning to our under water movie, we may note that the 

 holothurioids will leave only a few microscopic denticles and dermal 

 plates. The marine worms, if they are tube builders, will leave their 

 tubes. As for the clams and pectens, some of them will leave their 

 shells, although most of the shells will be crushed by octopuses and 

 other scavengers and the small bits will be passed repeatedly through 

 the bodies of marine worms in an effort to squeeze the last milli- 

 gram of nutrient material from them. Some of the bivalves will 

 settle in the mud, and even if the shells disintegrate, neat mud casts 

 will be left. In the Southwest mud casts are the most common 

 fossils. 



The foraminifera, ostracods, diatoms and other minute organisms, 

 which the picture does not show us but which we know are there, 

 will likely leave a record, provided they have hard tests. Many 

 thick deposits of shales exhibit no fossils which may be seen by 

 the unaided eye, but are teeming with microfossils, principally 

 foraminifera. 



