Mr. J. S. Bowerbank on the Siliceous Bodies of the Chalk. 249 



corrugated and torn spore-membrane. I have not succeeded in 

 discovering any purpose whatever in it, striking as the appear- 

 ance is. 



All these observations on the formation of spores confirm the 

 general results which H. von Mohl laid down in his memoir on 

 the development of the spores of Anthoceros Icevis, Linnsea, 1839, 

 vol. xiii. p. 273—290. 



1 . Four spores are always developed in a mother-cell. 



2. Previously to their development , a granular fluid matter is 

 contained in the mother-cell. Here it may be added, that this same 

 is formed of the dissolved cytoblasts. 



3. The four spores are formed at the same time, and certainly 

 not, as Mirbel believed, by the mechanical division of the cell-mass 

 into four parts by septa, these septa proceeding from the membrane 

 of the mother-cell, but in an independent manner. To this it may 

 be added, that actual cytoblasts are simultaneously produced in 

 the cell. 



The chief conclusion therefore is, that the process of spore- 

 formation does not differ from the formation of cells through cyto- 

 blasts. Psilotum cannot be too highly recommended for the ob- 

 servation of all these facts, as we here possess extraordinarily 

 large mother-cells which allow all the alterations in their interior 

 to be perceived with the greatest distinctness. 



Diversity in the peculiarities of the formation of spores in 

 Psilotum from that in Anthoceros and other Cryptogamic plants, 

 is of course owing to family and generic differences. 



[To be continued.] 



XXXI. — On the Siliceous Bodies of the Chalk and other Forma- 

 tions, in reply to Mr. J. Toulmin Smith. By J. S. Bowerbank, 

 F.R.S.&c. 



In the last January Number of the ' Annals and Magazine of 

 Natural History ' there are some observations by Mr. J. Toulmin 

 Smith on the Formation of the Flints of the Upper Chalk, in 

 which the author combats certain conclusions of mine published 

 in the ' Transactions of the Geological Society,' vol. vi. new series, 

 p. 181, relative to the spongeous origin of the flinty bodies of the 

 chalk, greensands, and oolites. Had the differences between the 

 author and myself been merely matters of opinion, I should not 

 have occupied your valuable pages on the present occasion, espe- 

 cially as he has declared* that "it is not his intention to dis- 

 pute the particular facts stated by myself as applying to the cases 



* Page 2. 

 Ann. ^ Mag. N. Hist. Vol.xix. 18 



