16 FOSSIL MEDUS 2. 
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 medusz 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 Pl. 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 medusee were cal- 
cified and preserved it is difficult to understand. If they were as firm of 
substance as the recent Discomedusze, 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, ete., 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 medusze 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 (Pl. 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 medusze or the borings of annelids. 
