Mineralogy and Geology. 



281 



related to those which I wish to present. That the calcareous shells of 

 the Polythalamia are sometimes replaced by silica, appears to have been 

 first noticed by Ehrenberg, who, in a note translated by Mr. Weaver, and 

 published in the L., E. and D. Philosophical Journal for 1841, (vol. xviii, 

 p. 397,) says : — 



" 1 may here remark that my continued researches on the Polythalamia 

 of the Chalk, have convinced me that very frequently in the earthy coat- 

 ing of flints, which is partly calcareous and partly siliceous, the original 

 calcareous shelled animal forms have exchanged their lime for silex with- 

 out undergoing any alteration in figure, so that while some are readily 

 dissolved by an acid, others remain insoluble ; but in chalk itself, all 

 similar forms are immediately dissolved." 



The first notice of casts of the cells and soft parts of the Polythalamia 

 was published by myself in the American Journal of Science for 1845, 

 vol. xlviii, where I stated as follows : — 



"The specimens from Fort Washington presented me with what [ be- 

 lieve have never been before noticed, viz: distinct casts of Polythalamia. 

 That these minute and perishable shells should, when destroyed by chem- 

 ical changes, ever leave behind them indestructible memorials of their 

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

 abundant arid easily to be recognized in some of the Eocene Marls from 

 Fort Washington." This notice was accompanied by figures of well- 

 defined casts of Polythalamia (1. c. pi. iv, fig. 30, 31). 



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

 their soft parts, preserved in flint and chalk, and communicated an ac- 

 count of them to the Royal 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 

 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, (see Monatsbericht, 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 (Steinkernbildung) mostly of Polythalamia, 

 was stated in July of the preceding year." He then refers to the Num- 

 mulite Limestone of Traunstein in Bavaria, as rich in green opal-like 

 casts (Opalsteinkernen) of well-preserved Polythalamian forms, and men- 

 tions 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 Polythalamia, and they often 

 foim 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 



SECOND SERIES, VOL. XXII, NO. 65. — SEPT., 1866. 



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