636 



MICROSCOPIC GEOLOGY. 



which pass from one surface to the other like those of dentine. 

 These tubes are shown, as divided lengthways by a vertical sec- 

 tion, in Fig. 338 [a, a) ; whilst the appearance they present when 



Fig. 338. 



Porlion of a Ihiii seclioii of Nummulites Itevigaia^ laken in the direcUon of the preceding, hi;(lily 

 magnified to show the minute structure of tlie shell : — a. a, portions of the ordinary shell-substance 

 traversed by parallel tubuli ; by b, portions forming the mur-iinal wall, traversed by diverging and 

 larger tubuli; c, one of the chambers laid open; df d, d, pillars of solid substance not perforated 

 by tubuli. 



cut across in a horizontal section is shown in Fig. 339, the 

 transparent shell-substance, a, a, a, being closely dotted with 

 minute punctations which mark their orifices. In that portion 

 of the shell, however, which forms the margin of each whorl 

 (Fig. 338, b, b), the tubes are larger, and diverge from each 

 other at greater intervals ; whilst at certain other points, d, d, d, 

 the shell-substance is not perforated by tubes, but is peculiarly 

 dense in its texture, forming solid pillars which seem to strengthen 

 the other pai-ts. In ISTummulites whose 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 lesa worn down, are left as jirominerices ; 

 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 separates them, and the 

 margin of the preceding whorl that foi'ms their inner boundary ; 

 this passage is sometimes a single large broad aperture, but is 

 more commonly formed by the more or less complete coalescence 

 of several separate perforations, as is seen in Fig. 337, b. Such 

 marked differences in this respect are observable in the several 

 parts of one and the same specimen, that it is obvious that veiy 

 little account should be taken of dilFerences in the form of aper- 

 ture, as affording specific or generic distinctions among Fora- 

 minifera of this type. But besides the foregoing means of com- 

 munication, by which the segments of sarcode included in the 

 inner chambers were enabled to continue receiving supplies of 

 nutriment, we meet in Nummulites with a remarkable develop- 

 ment of that system of "interseptal" canals, one of the most 

 characteristic examples of which among recent Foraminifera is 



