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municatcd an account of them to the Ivoyal Society of London, 

 in May, 1846. In this paper he speaks of the chambers of 

 Polythalamia as being frequently filled with chalk, flint, and 

 silicate of iron. (Phil. Trans., 1846, p. 466.) To Ehrenberg, 

 however, appears to be due the credit of first distinctly announc- 

 ing the connection between the Polythalamia and the forma- 

 tion 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, (see 

 Monats-Bericht, Berlin, February, 1855,) 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 (Steinkernbil- 

 dung,) mostly of Polythalamia, was stated in July of the pre- 

 ceding year." He then refers to the Nummulite Limestone of 

 Traunstein, in Bavaria, as rich in green opal-like casts (Opalstein- 

 kernen) of well-preserved Polythalamian forms, and mentions 

 them as also occurring, but more rarely, in the Glauconite Lime, 

 stones of France. He then proceeds to give an account of his 

 detection of similar casts in 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 yellowish, and 

 under a lens appears spotted with green. These green spots are 

 the Greensand casts of the Polythalamia, and they often form as 

 much as one third of the mass. By solution in dilute chloro- 

 hydric acid, the greensand grains are left, mixed with quartzose 

 sand, and with a light yellowish jnud. The latter is easily re- 

 moved 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 fre- 

 quently found spiral, or corkscrew-like bodies, which Ehrenberg 

 considers as casts of the shells of young mollusks. 



AVith reference to the perfection of these casts of the Polythal- 

 amia, 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 



