6 FOSSIL MEDUSA. 



It may be urged that the plaster sets very quickly, and that sediment 

 deposited in the sea would require so much time for its consolidation that 

 an organism as delicate as a medusa would be decomposed and crushed. 

 In this connection I wish to record an observation made at the inlet west 

 of Noyes Point, Rhode Island, which indicates that sediments may consoli- 

 date and harden very rapidly under favorable conditions. A deposit, formed 

 of fine sand and silt, hardened to such an extent during the time that 

 elapsed between its deposition by the outflowing tide and the return of the 

 tide that it was broken up by the waves of the latter so as to form a brec- 

 ciated layer, the fragments of which remained, often with sharp edges, after 

 the ebb of the tide on the following morning. That sediments may set and 

 harden quickly under water is known to those who have waded or dredged 

 in shallow waters in protected bays and inlets. 



Mr. H. Archer notes, in the following words, the mode of occurrence of 

 a species that is referred to I'ohjclonia frondosa by Prof. Alexander Agassiz : 



A few years ago I was quartered for some time at Port Royal, Jamaica, and in 

 the channels between the mangroves I observed what I at first thought were Actinias 

 of large size on the muddy bottom, in about 8 feet of water. They were very numer- 

 ous. I stirred one up with the boat hook, and was surprised to find it was a medusa 

 turned upside down. On being disturbed it lazily contracted its umbrella in the usual 

 manner and settled down again in the mud as before. The species was about a foot 

 in diameter of umbrella, and dirty white in color. I never saw them swimming in the 

 mangrove creeks, though I was frequently out in a boat, and they were at all times 

 common on the bottom, lying as described. Some time afterwards I saw what seemed 

 to be the same species at St. Georges Bay, a small island about 10 miles from Belize, 

 Honduras. It was lying in the same position on the mud amongst the mangroves, in 

 about i feet of water. I poked several up with a stick, and they slowly swam for a 

 short distance and again settled down on their umbrellas. I believe it to be really 

 the habit of the species to lie on its back, as it were, and it is interesting to find 

 another kind in the East acting similarly. Mangrove swamps are extensive in the 

 vicinity of Singapore, but I have not noticed any medus;e here in that position, pos- 

 sibly because there is a considerable tide which leaves the mud bare at low water. 1 



Professor Agassiz, in commenting on Mr. Archer's note, said: 



The medusa mentioned by Mr. Archer in Nature, Vol. XXIV, p. 307, is undoubt- 

 edly Polyclonia frondosa Ag., figured in the Contributions to the Natural History of 

 the United States. This medusa was already known to Pallas, who described alco- 

 holic specimens sent him from the West Indies by Drury. It is stated by Agassiz to 

 be quite common along the Florida Keys. I have myself observed it in great abun- 



1 Nature. Vol. XXIV, 1881, p. 307. 



