MIDDLE CAMBRIAN. ff 
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 Actinize 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.! 
Prof. W. K. Brooks informs me that both Polyclonia and Cassiopea 
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 medusz 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 medusze 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 medusze 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. 
1Nature, Vol. XXIV, 1881, p 509. 
