FOSSIL MEDUSJ3. 



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 medusae, 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 Pis. IV and XXIII.) 



An examination of the character and habits of some of the Discome- 

 dusae shows that, from their mode of occurrence, the Middle Cambrian 

 fossil medusae 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 medusae was not far from shore is proved by the 

 character of the sediments. The latter, we know, were deposited in the 

 Appalachian sea 1 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 medusae that were resting on 

 the bottom or floating in the water. 



The endoderm of the recent medusae, 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. 



