24 DE. CAEPENTEE’S EESEAECHES ON THE EOEAIVIINIEEEA. 
orifices of the tubuli are separated by a very debcate areolation (fig. 95), as ^ the whole 
substance were composed of an aggregation of prisms with a tubulus m the centre o 
each And this circumstance appears to me to afford the explanation of an observaUon 
which I have elsewhere recorded in regard to certain fossil specmens ot Mmmuhtes 
(loc cit ) —namely, that where the characteristic tubular structme is wantmg, an appe^- 
ince of prismatic Structure is occasionally presented.; for it is not by ^y means diffi- 
cult to conceive that the same molecular change which obliterates the one feature, 
should bring the other into increased distinctness. The size of the tubuh of O^ercu- 
Ima is much greater at and near the inner surface of the walls of the cfiambei^ t an it 
is at a little distance from them, as will be seen by comparing figs. 1 and 3 ol Tlate . 
with figs. 2 and 4 ; so that when a thin lamina from the innermost part of the wa is 
examined under a high magnifying power, the tubes are seen almost to fill the areolee, 
as is shown in Plate IV. fig. 9 «, which is drawn on the same scale as h. 
156. As is NummulUes and Gyclodypeus, the shell-substance over the septa is not 
penetrated by tubuli, and is consequently far more transparent than the rest; and it is 
in consequence of this difference in texture, that the septal bands are so strongly marked 
on the external surface, as well as in sections parallel to it (Plate I\. figs. 1 , , 
Plate VI. figs. 1, 2). The same is true, also, of the substance of the tubercles, whether 
occurring as elevations of the septal bands, or upon intermediate parts of spnal 
lamina In the latter case, the tubercles are seated upon inverted cones of the like 
substance, which do not usually reach down to the inner lamina, so that the internal 
surface presents the orifices of the tubuli regularly disposed overtire interior of the 
chambers (as seen in Plate IV. figs. I, 3), even where sections passnig at a httle distance 
from these show rounded or elongated spots of transparent substance (figs. 2, 4), around 
which the tubuli are crowded together, as if they had been displaced by its inter- 
position. Such spots occasionally present themselves in the ordinary type of Operculma 
as seen in Plate IV. fig. 10 ; but it is of course in sections of the strongly-tiiberculated 
variety that we find them most marked, and this especially in the investing layers of the 
central region, where, from the size and approximation of the tubercles the spaces 
between them are rendered almost opaque by the crowding together of the tubuli 
(Plate IV. fig. II). The tubuli are generally seen, moreover, to be somewhat more 
crowded in the neighbourhood of the septal bands.— The greatest local development 
of the non-tubular substance is seen in those varieties which have a large elevated 
tubercle in the centre (Plate III. figs. 3-5) ; a section of such a variety has been shown 
in Fig. VII. D, where it is seen that this tubercle springs from the spual lamina of the 
* As MM d’Aechiac and Haimb, in referring to my statement as to the appearance of prismatic stiuc 
tore in Nummulites, wonld appear to entertain some donbt as to the coinectness of 
p 61 of their Monograph), I think it right not merely to say that fig. 12, PL IV. of my Memon- on Num- 
LlLs i! a most exact representation of a section stfil in my possession, but also to mention that the ve. 
tical fracture of well-preserved specimens of N. Imvigata often presents an edge so strongly resem mg 
oftrp^ shelT-substance M when fractured in the same direction (though on a --h smafi^ 
Lie), L to have led experienced microscopic observers to a belief in them similarity, beiore my discover! 
of the minutely tubular structure of Nwrimulites. 
