70 HISTORICAL PALEONTOLOGY. 



The concentrically-laminated mass of Eozoon is composed 

 of numerous calcareous layers, representing the original skele- 

 ton of the organism (fig. 23, b*). These calcareous layers serve 



to separate and de- 



system just spoken 



of, is bounded both above and below by a thin lamina which has 

 a structure of its own, and which may be regarded as the proper 

 shell-wall (fig. 23, a a). This proper wall forms the actual lin- 

 ing of the chambers, as well as the outer surface of the whole 

 mass ; and it is perforated with numerous fine vertical tubes 

 (fig. 24, a a), opening into the chambers and on to the sur- 

 face by corresponding fine pores. From the resemblance of 

 this tubulated layer to similar structures in the shell of the 

 Nummulite, it is often spoken of as the " Nummuline layer." 

 The chambers are sometimes piled up one above the other in 

 an irregular manner; but they are more commonly arranged 

 in regular tiers, the separate chambers being marked off from 

 one another by projections of the wall in the form of parti- 

 tions, which are so far imperfect as to allow of a free communi- 

 cation between contiguous chambers. In the original condi- 

 tion of the organism, all these chambers, of course, must have 

 been filled with living matter; but they are found in the present 

 state of the fossil to be generally filled with some silicate, such 

 as serpentine, which not only fills the actual chambers, but has 

 also penetrated the minute tubes of the proper wall and the 

 branching canals of the intermediate skeleton. In some cases 



