98 



THE MICROSCOPE AND ITS REVELATIONS. 



surfaces have been much exposed to attrition, it commonly happens that 

 the pillars of the superficial layer, being harder than the ordinary shell- 

 substance, and being consequently less worn down, are left as promi- 

 nences, the presence of which has often been accounted (but erroneously) 

 as a specific character. The successive chambers of the same whorl 

 communicate with each other by a passage left between the inner edge 



of the partition that sepa- 



Fl - 335 - rates them, and the ' mar- 



ginal cord ' of the preced- 

 ing whorl; this passage is 

 sometimes a single large 

 broad aperture, but is more 

 commonly formed by the 

 more or less complete co- 

 alescence of several separ- 

 ate perforations, as is seen 

 in Fig. 334, b. There is 

 also, as in Operculina, a 

 variable number of isolated 



Portion of a thin Section of Nummulina lavigata, taken in P ores in most f tne Septa, 



the direction of the preceding, highly magnified to show the forming a SCCOndaiT means 



minute structure of the shell: a, a. portions of the ordinary . ,- r , 



shell-substance traversed^ by parallel tubuli; 6, 6, portions 01 Communication between 



the chambers. The Ca- 

 nal-system of Nummulina 

 seems to be distributed 

 upon essentially the same plan as in Operculina; its passages, however, are 

 usually more or less obscured by fossilizing material. A careful exami- 

 nation will generally disclose traces of them in the middle of the parti - 



forming the marginal cord, traversed by diverging and larger 

 tubuli; c, one of the chambers laid open; d, d. d, pillars of 

 solid substance not perforated by tubuli. 



FIG. 336. 



FIG. 337. 



Portion of Horizontal Section of 

 Nummulite, showing the structure of 

 the walls and of the septa of the 

 chambers: a, a. a, portion of the wall 

 covering three chambers, the puncta- 

 tions of which are the orifices of tubuli ; 

 &, 6, septa between these chambers, 

 containing canals which send out late- 

 ral branches, c, c, entering the cham- 

 bers by larger orifices, one of which is 

 seen at d. 



Internal cast of two of the chambers, 

 a, a, of Nummulina striata, with the 

 network of Canals, b, 6, in the marginal 

 cord, communicating with canals pass- 

 ing between the chambers. 



tions that divide the chambers (Fig: 336, b, b), while from these may be 

 seen to proceed the lateral branches (c, c), which, after burrowing (so to 

 speak) in the walls of the chambers, enter them by large orifices (d). 

 These ' interseptal ' canals, and their communication with the inosculat- 



