4 FOSSIL MEDUS 2. 
CONDITION AND MANNER OF PRESERVATION. 
In physical appearance the nodules that weather out of the shale are 
externally of a dull-yellow or ocher color, and, when unaltered, of a dark 
color inside. Many of the nodules are slightly calcareous, and when the 
calcareous matter has been dissolved and oxide of iron developed, there 
remains a red, yellow, or dark, siliceous, ironstone-like nodule. In these 
nearly all traces of the medusze, except the outer form, are lost. The body 
of the medusa is usually preserved in the sections as a dark-gray or black 
mass, often with many oolitic-like grains, with the filling of the umbrella 
beneath the body and arms of a lavender or yellowish-lavender color. 
(See Pls. IV and XXIII.) 
An examination of the character and habits of some of the Discome- 
dusze shows that, from their mode of occurrence, the Middle Cambrian 
fossil medusze appear to have had something of the same habits as the 
recent Polyclonia and Cassiopea in living on a firm, muddy bottom in large 
numbers. The associated fossils indicate no great depth of water; and 
that the habitat of the medusze was not far from shore is proved by the 
character of the sediments. The latter, we know, were deposited in the 
Appalachian sea’ in an area where calcareous and argillaceous muds and 
alternating beds of sand were accumulating. These conditions were favor- 
able to the more or less rapid burial of the medusz that were resting on 
the bottom or floating in the water. 
The endoderm of the recent medusz, Polyclonia and Cassiopea, is tough 
and strong, and I obtained a very good cast in plaster of a small alcoholic 
specimen in which the general form and oral arms are fairly well shown. 
Prof. Louis Agassiz states that Aurelia flavidula, after the spawning 
period, is often seen in large numbers floating upon the water. There has 
been a thickening of the tissues by an increased deposition of animal sub- 
stance. The disk of the animal has become thin and almost leathery, and 
it is more elastic, though at the same time more brittle, than it was before. 
The tentacles are for the most part gone, as well as the eyes, and this 
decomposition of the margins extends so far that even the marginal 
tube and parts of the anastomoses have disappeared. The fringes along 
'The North American continent during Cambrian time: Twelfth Ann. Rept. U. S. Geol. 
Survey, Part I, 1891, p. 536. 
