. limestone is yellowish, and under a lens appears spot 
SE ele aie ON Ms dna ei a Le 
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 :— 
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, 
lieve have never been before noticed, viz: distinct casts of Polythalamia. 
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 
ington.” This notice was accompanied by figures of well- 
defined casts of Polythalamia (I. ¢. pl. 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- 
€ speaks of the chambers of Polythalamia as being frequently 
filled with chalk, flint, and silicate of iron. (Phil. Trans., 1846, p. 466.) 
To Ehrenber , however, appears to be due the credit of first distinctly 
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 
Slodon from Alabama, (see Monatsbericht, Berlin, February, 1855,) 
“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, 
4 4 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 
Se (Opalsteinkernen) of well-preserved Polythalamian forms, and men- 
Hons them as also occurring, but more rarely, in the Glauconite Lime- 
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 sc 
green spots a Greensand casts of Polythalamia, and they often 
uch as — et mass. By solution in dilute chloro- 
hydric acid, the greensand grains are left, mixed with quartzuse sand, and 
_ SECOND sERIEs, vot. XXII, NO. 65,——SEPT., 1956. 
36 
Mineralogy and Geology. 281 
