304: ANNUAL OF SCIENTIFIC DISCOVERY. 



have never been before noticed, viz. distinct casts of Polytkalamia. That 

 these minute and perishable shells should, when destroyed by chemical 

 changes, ever leave behind them indestructible memorials of their existence 

 was scarcely to be expected, yet these casts of Polythalamia are abundant 

 and easily to be recognized in some of the Eocene Marls from Fort Washing- 

 ton." 



Dr. Mantell also noticed the occurrence of casts of Polythalamia and their 

 soft parts, preserved in flint and chalk, and communicated an account of 

 them to the Royal Society of London. To Ehrenberg, however, appears to 

 be due the credit of first distinctly announcing the connection between the 

 Polythalamia and the formation of Greensand, thus throwing the first light 

 upon the origin of a substance which has long been a puzzle to geologists. In 

 a notice given by this distinguished observer upon the nature of the matrix 

 of the bones of the Zeuglodon from Alabama, he says : 



" That Greensand, in all the numerous relations in which I have as yet 

 examined it, has been recognized as due to the filling up of organic cells, as 

 a formation of stony casts mostly of Polythalamia, was stated in July of the 

 preceding year." He then refers to the Nummulite Limestone of Traunstein, 

 in Bavaria, as rich in green opal-like casts of well-preserved Polythalamian 

 forms, and mentions them as also occurring, but more rarely, in the Glauconite 

 Limestones of France. He then proceeds to give an account of his detection of 

 similar casts hi the limestone adhering to the bones of the Zeuglodon from 

 Alabama, and states that this limestone abounds in well-preserved brown, green, 

 and whitish stony casts of recognizable Polythalamia. This limestone is yellow- 

 ish, and under a lens appears spotted with green. These green spots are the 

 Greensand casts of Polythalamia, and they often form as much as one third of the 

 mass. By solution in dilute chlorohydric acid, the greensand grains are left, 

 mixed with quartzose sand, and with a light yellowish mud. The latter is 

 easily removed by washing and decantation. The casts thus obtained are so 

 perfect that not only the genus, but often the species of the Polythalamia, can 

 be recognized. Mingled with these are frequently found spiral, or corkscrew- 

 like bodies, which Ehrenberg considers as casts of the shells of young mollusks. 



With reference to the perfection of these casts of the Polythalamia, and the 

 light they throw upon the structure of these minute animals, Ehrenberg 

 remarks : 



" The formation of the Greensand consists in a gradual filling up of the 

 interior space of the minute bodies with a green-colored, opal-like mass, which 

 forms therein as a cast. It is a peculiar species of natural injection, and is 

 often so perfect, that not only the large and coarse cells, but also the very finest 

 canals of the cell walls, and all their connecting tubes, are thus petrified, and 

 separately exhibited. By no artificial method can such fine and perfect 

 injections be obtained." 



Having repeated the experiments of Ehrenberg upon the Zeuglodon Lime- 

 stone, I can confirm his statements in every particular, and would only add, 

 that besides the casts of Polythalamia and small spiral mollusks, there is also 

 a considerable number of green, red, and whitish casts of minute anastomosing 

 tubuli, resembling casts of the holes made by burrowing sponges (Cliona) and 



