282 THE AMERICAN MONTHLY [Sept 



stratification, frequently exhibit very curiously marked 

 sections. Hence the names; beetle stones, turtle stones, 

 Ludi helmontii and the like. The fossil species of the 

 Island of Sheppy indicate a much more tropical climate 

 tlian the Eocene flora of France. The coast was sunk 

 lower then and was warmer. The larger fossils are more 

 tropical and the Bacillaria are more tropical. We find 

 specimens of Arachnoidiscus there. It is comparatively 

 a scarce form in that region. One specimen has been 

 seen in England and one in Ireland. It is common in 

 the Pacific states being brought to that coast by the Kura- 

 Sigra or Janauss current from Japan where it is com- 

 mon also. 



Cleaning the London clay carefully and viewing it by 

 means of the microscope transparently, it is seen to con- 

 tain sparsely certain discs that are black; and looking 

 them over some will be seen semi-transparent and so 

 fashioned as to show that they are diatoms. They were 

 first Coscinodiscus asteromphalus, — little discs with 

 hexagonal markings all over them. The London clay 

 diatoms show the structure much more clearly than can 

 usually be seen in transparent specimens. The cell mem- 

 brane, which is colloid silica is removed and an internal 

 cast of the cells shown. When they arc viewed by front 

 view they are seen to be curved outward on the interior 

 and exterior, which is to say they are almost spherical. 

 The specimen looks as if the disc consisted of a series of 

 spherical balls set along side of one another. The mater- 

 ial of which the black substance is composed is pyrite 

 iron pyrites or sulphide of iron, formed by iron sulphate 

 in the salt water in which the diatoms occurred acting on 

 the organic matter of the diatoms, the pi'ot(,)[)lasm, which 

 was decomposed, the oxygen l)eing set free and the iion 

 and sulphur thrown down as sulphide of iron. The dia- 

 toms can be seen when viewed with reflected liglit to be 

 glistening, almost gold-colored, particles. A ring look- 



