MIDDLE CAMBRIAN. 7 



dance at the Tortugas, in the moat of Fort Jefferson, and in the mud flats to the north 

 of Key West. They occur there in from 3 to 6 feet of water, the disk resting upon the 

 bottom, the tentacles turned upwards; the disk pulsates slowly while they are at rest. 

 Their habits when disturbed are well described by Mr. Archer. The young sometimes 

 swim near the surface, and are far more active than larger specimens. When kept in 

 confinement they also creep slowly over the ground by means of their tentacles, or, 

 raising themselves sometimes edgeways against the sides of the dishes, remain sta- 

 tionary for a considerable time. The resemblance of Polyclonia, when at rest upon 

 the bottom, to large Actiniae with fringed tentacular lobes, such as Phythactis, is very 

 striking, The peculiar habits of Polyclonia were noticed by Mertens in a species 

 named by Brandt P. Mertensii in 1838, and found at the Carolines. 1 



Prof. W. K. Brooks informs me that both Polyclonia and Cassiop.ea 

 occur in abundance near Port Royal, Jamaica, and that there is no way 

 of telling which genus Mr. Archer observed, as their habits are almost 

 identical, and they occur in similar localities. 



The mode of occurrence of the fossil medusae in the Middle Cambrian 

 (p. 3) suggests at once the habit of living on a muddy bottom in great 

 numbers. The same is also true of the Lower Cambrian forms from the 

 roofing slates of eastern New York. It is only by their having some such 

 habit that I can account for the preservation of the medusae in such great 

 numbers and in such condition as they are found in the shales of northern 

 Alabama. 



The conditions most favorable to the preservation of a medusa of the 

 character of Polyclonia and Cassiopea, and consequently the Middle Cam- 

 brian species, appear to be rapid burial and consolidation of the sediment, 

 not by exposure between tides, but entirely beneath the water. If the 

 medusa were buried in such a sediment its watery contents would not be 

 drained off and produce a collapse before the sediment that penetrated into 

 the interior and settled about the exterior had time to harden. There is no 

 a priori reason why the external form and the radial, intestinal, and other 

 interior canals should not be preserved under such favorable conditions. 

 Not one in a hundred of the fossil specimens, however, show traces of any 

 structure within the body, and, so far as known at present, the particularly 

 favorable conditions required, even for this partial preservation of the 

 structure of the medusa' in a fossil state, were confined during geologic 

 time to the vicinity of the spot in the Cambrian sea that is now occupied 

 by the township of Cedar Bluff, Cherokee County, Alabama. 



■Nature, Vol. XXIV, 1881, p 509. 



