662 GEOLOGY. 



and they most abound in the limestones,, some of which are largely 

 made up of their remains, as the encrinital Tmestone (Fig. 349). They 

 will be subjects of frequent comment and illustration in the historical 

 chapters. 



The contribution of the Vermes. — Most of the worms are ill adapted 

 to fossilization and are not known in the fossil form. The segmental 

 worms of the sea, the annelids, however, left some traces of themselves 

 in tubes and borings and in tracks and sometimes by fossil jaws and 

 teeth. They range from the earliest fossil-marked horizons onward, 

 but seem to have always been an inferior group. 



The contribution of the Molluscoidea. — This group includes the bryo- 

 zoans, whose fossil products closely resemble the minute-celled corals, and 

 the brachiopods, whose shells closely resemble those of the molluscs. 

 Both are calcareous and make important contributions to the formation 

 of limestone (Fig. 350). A few brachiopods secrete calcium phosphate 

 instead of calcium carbonate. Both classes have a great geologic range 

 and their fossils are valuable aids in identifying and correlating forma- 

 tions. Probably the brachiopods are more utilized for this purpose 

 than any other single class. They are the symbol of conservatism and 

 persistence, ranging from the Cambrian to the present time, and em- 

 bracing some forms that have scarcely changed to the extent of generic 

 difference in that time. 



The contribution of the Mollusca. — The molluscs have also ranged 

 from the earliest well-recorded times, and some divisions, as the pelecy- 

 pods (lamellibranchs, embracing clams, oysters, etc.) and gastropods 

 (snails, etc.), have undergone no very marked change beyond a rather 

 ample and progressive development; but others, as the cephalopods 

 (nautilus, squids, cuttle-fish, etc.), mark out the progress of the ages: 

 by distinct and striking changes of form. Their shells are chiefly cal- 

 careous and they have contributed materially to the formation of lime- 

 stone. Muddy and sandy bottoms are, however, more congenial to the 

 pelecypods and gastropods than to the corals, crinoids, and many other 

 limestone-forming types, and hence fossils of these molluscs frequently 

 abound in shales and sandstones and give them a calcareous element. 

 In sandstones, however, the calcareous matter is often dissolved out 

 and only the casts of the shells remain. The molluscs will be much cited 

 and illustrated in the historical chapters. 



The contribution of the Arthropoda. — This group embraces the 



