MIDDLE CAMBRIAN, 5 
the margins of the oral appendages gradually drop off, and with them parts 
of the arms themselves, especially toward the extremities, which become 
blunt. Professor Agassiz says “the manner in which stranded medusze are 
sometimes covered in hot, dry, and windy days by floating sand and 
molded in it, explains the possibility of acalephs in the fossil state. The few 
specimens found in the fine-grained limestones of Solenhofen were probably 
preserved in this way.”! 
In the fall of 1893 I observed great numbers of specimens of Aurelia 
that had been thrown up onto the sand of the New Jersey beach, in the 
condition described by Professor Agassiz. They collapsed, however, after 
a few hours’ exposure to the sun and wind. 
While experimenting, in 1895, with some living specimens of Aurelia 
Jlavidula in the Indian River, on the coast of Florida, a large specimen was 
thrown into the quiet, shallow water at my feet, where it remained for some 
little time, when I observed that it was lying on its back and that the arms 
had dried and shrunken in the bright sunlight, and the body had swollen 
so that the genital openings and the mouth were completely lost. On pick- 
ing up the medusa, I found it to be firm and hard, and when tossed on the 
wharf it did not break or tear. The shrunken arms were tough, and it 
required considerable force to pull them apart. This condition might pos- 
sibly explain how a medusa, when killed by being overwhelmed by a 
sudden incursion of muddy sediment into the water in which it was living, 
might retain its shape a sufficient length of time to have the sediment settle 
closely about it and its cavities and to solidify, so as to make a mold of its 
exterior form. 
I found no difficulty in securing plaster casts of Aurelia. A bed of 
soft plaster was prepared and a living medusa taken directly from the water 
and laid thereon and at once covered by pouring a thin mixture of plaster 
over it so as to completely bury it in the mass. When the plaster had set, 
the water in both plaster and medusa was removed by evaporation and an 
opening made into the cast. It was then found that the plaster had pene- 
trated the genital openings and cavities and the mouth and gastric cavity 
(so far as the latter was open), and that the cast showed the details of the 
form of the animal in a very beautiful manner. Photographs of some of 
the casts are shown on Pls. XXX and XXXI. 
‘Contributions to the Natural History of the United States of America, Vol. IV, 1862, p. 63. 
