290 On the Structure and Position of Eozoon Ganadense. 



The minute structure of Eozoon may be determined by the 

 microscopic examination either- of thin transparent sections, 

 or of portions which have been subjected to the action of 

 dilute acid, so as to remove the calcareous shell, leaving only 

 the internal casts, or models, in silex, of the chambers and 

 other cavities originally occupied by the substance of the 

 animal. Each of these modes of examination, as I have else- 

 where shown,* has its peculiar advantages ; and the combina- 

 tion of both, here permitted by the peculiar manner in which 

 the Eozoon has been fossilized, enables us to attain a complete- 

 ness of knowledge of its structure, such as is afforded by no 

 other fossil with which I am acquainted. For in well-pre- 

 served specimens, the shelly substance often retains its 

 characters so distinctly, that the details of its structure can be 

 even more satisfactorily made out, than can those of most of 

 the comparatively modern Nummulites. This arises from the 

 fact, that whilst the latter, when imbedded (as they usually 

 are) in a matrix of the same material, have been subjected to 

 calcareous infiltration, which has filled- up alike their minute 

 tubules and their larger canals, and has thus rendered the 

 shell- substance nearly homogeneous, these tubules and canals 

 have been filled up in Eozoon by a siliceous infiltration, which 

 does not coalesce with the substance of the shell, so that the 

 boundaries of the tubules and canals can be distinctly defined. 

 But what renders the condition of Eozoon so peculiarly 

 favourable for the investigation of its organic structure, is the 

 marvellous completeness with which the minutest extensions 

 of the sarcode-body of the animal are represented in decalci- 

 fied specimens by their siliceous models ; even the most delicate 

 pseudopodial threads, consisting of the softest and most transi- 

 tory form of living substance, which were put forth through pores 

 in the shell- wall of less than 1 , o 00 th of an inch in diameter, 

 being thus, as it were, perpetuated to all time ; and the 

 varieties of their course being exhibited, by what appear 

 under the microscope as most perfect models, in asbestiform 

 fibre, having this advantage over the most skilfully exe- 

 cuted works of human hands, that they are not imitations, 

 but the very threads themselves turned into stone by Nature's 

 cunning. For there can, I think, be no doubt that the 

 siliceous mineral found its way into the cavities of the shell, 

 not by mere mechanical infiltration occasioned by pressure 

 from without, but by a process of chemical substitution which 

 took place, particle hj particle, between the sarcode-body of 

 the animal and certain constituents of the ocean-waters, before 

 the destruction of the former by ordinary decomposition. 



* Memoir on Polystomella, in the Philosophical Transactions for 1860, pp. 

 538, 540 ; and Introduction to the Study of the Foraminifera, pp. 9, 10. 



