16 FOSSIL MEDUSJE. 



The inference that the nodule remained for some time in a relatively 

 plastic condition is proved by the presence, in several hundred sections, of 

 fine annelid borings that penetrate the nodules in all directions, cutting 

 through medusse and matrix alike (PL XXIII, figs. 10 and 11). This occurs 

 whether the specimen of medusa is a cast of its outer mold or of its body, 

 appendages, and interior parts. The casts of the borings are filled with 

 material differing from the surrounding matrix in color and composition, 

 and often it has dissolved and left minute openings. Many nodules have 

 vertical borings (Scolithus-like) passing through them. Sometimes there is 

 a hole or filling from bottom to top, and often only surface markings 

 remain, as shown by fig. 8 of PI. XVI. 



Were the nodules calcareous or siliceous as they were originally 

 formed? To the student of the processes of mineralization or fossilization 

 of organisms the view is at once suggested that the nodules were at first 

 calcareous and that the calcite was subsequently replaced by siliceous 

 matter brought in by silicifying solutions. Just how the medusse were cal- 

 cified and preserved it is difficult to understand. If they were as firm of 

 substance as the recent Discomedusse, Polyclonia and Cassiopea, and were 

 buried in mud that set quickly, a very good mold might result. The ani- 

 mal matter would have to be replaced rapidly by the calcite if the interior 

 canals, etc., were to retain their true relations to the umbrella lobes and 

 other parts. If, however, all of the interior parts were destroyed the mold 

 might be filled in by calcite and a cast of the exterior form of the medusa 

 result. Among the fossil medusse many specimens preserve the interior 

 canals, but such is not the case in a majority of specimens. Hundreds 

 occur in which there is not a trace of any of the interior parts (PI. XXIII, 

 fig. 9). If the original mineralization was by calcite the replacement by 

 silica, deposited from infiltrating silicifying solutions, could take place at 

 any time after the deposition of the calcareous matter. Usually such 

 replacement occurs long after the embedding sediment is hardened into 

 rock. If the silicification was by replacement of calcite, we must assume 

 that the radial and other canals were filled and preserved at the time of the 

 deposition of the calcite, also the numerous annelid borings that must have 

 also existed in the medusa, and the surrounding material that gave shape to 

 the nodule. Such a theory is scarcely tenable with any of the specimens 

 showing the interior canals of the medusas or the borings of annelids. 



