CORNIFEROUS PERIOD. 



271 



facts show that hornstone is analogous to flint in origin as well as 

 in its mode of occurrence : the two are the same in composition 



Fig. 441. 



Fucoides Cauda-Galli 



(page 55). Figs. 441 A a-d represent the sporangia (spore-capsules, 

 or receptacles containing the germinative cells) of Desmidieae, or 



Microscopic Organisms in Hornstone. — Figs, a-i ancH-w, Protophytes; j, k, Spiculacf 

 Sponges ; o, p, fragments of dental apparatus of Gasteropods. 



Desmids, — closely resembling organisms from flint called Xan- 

 thidiahj Ehrenberg; — e, a Desmid having the usual division into 

 halves ; /, g, Desmids consisting of several cells ; i, a Diatom : — 

 figures magnified about 225 diameters. The sizes of the specimens 

 figured vary from l-500th to l-5000th of an inch : diameter of 

 fig. a, l-500th in. ; of d, e, l-1500th ; of i, l-1000th ; of cells in/, g, 

 l-7000th by l-5000th. Desmids, like the Diatoms, are microscopic 

 plants, consisting of one or a few cells ; but they secrete little or 

 no silica, and have a pale-green color. The hornstone also contains 

 numerous rhombic crystals, probably of calcite, from l-500th to 

 1-lOOOth inch in diameter. 



