114 AN INTRODUCTION TO THE STUDY OF FOSSILS 



it was connected by means of a tube with the central canal 

 running the length of the stalk. Unlike Sertularia, however, 

 it was not attached to the sea-bottom in shallow water, but 

 floated freely at its surface suspended from a float-like disk 

 (Fig. 40,/.), somewhat like forms included with the hydrozoan 

 order Siphonophora. 



Fig. 40. — Diplograptus foliaceus Murchison, from the Utica (Ordovician) shale 

 of New York. A, a compound colony of graptolites. Natural size. B, a 

 young graptolite just emerged from the reproductive bladder (r.). Natural size. 

 C, the living hydrozoon, Sertularia pumila ( X 2), for comparison of external 

 forms, fl., the supporting float-like disk ; r., reproductive bladders in three of 

 which are young graptolites (siculae, s.) ; ///., thread supporting the colonies 

 (c, c'.) ; c, side view of colony showing the two rows of cups; c' ., front view 

 showing the width of the cups. -4, B, slightly modified from Ruedemann. 



As in Sertularia, the protective covering was of chitin, but 

 its partial decomposition, due to its great age, aided probably 

 by the heat engendered by pressure of the overlying sediments, 



