294 FAMILY NUMMULINIDA. 



horizontal roof and floor of the chamber ; but elsewhere they diverge from one another, 

 leaving an interseptal space, which is partly filled up by an interposed lamina of shell- 

 substance, but is partly occupied by the inferscpfal canals to be presently described 

 (fig. 5, h, h, li). A thicker space of the same kind is in like manner left between the proper 

 walls of the chambers forming one annulus, and those of the chambers forming the annuli 

 internal and external to it ; this space is almost entirely filled up by a shelly deposit, the 

 interseptal canals which pass between the successive annuli being less numerous than those 

 which run between the chambers of the same annulus (H 498). This intervening deposit 

 constitutes the " intermediate " or " supplemental " skeleton of this type. As in Orbifolites, 

 the chambers of each annulus usually alternate in position with those of the annuli internal 

 and external to it. But this is by no means constantly the case ; since additional chambers 

 are ' interpolated' here and there, so as to increase the number according to the augmented 

 diameter of the annulus ; and such an interpolation disturbs the regular arrangement of the 

 neighbouring chambers. The adjacent chambers of the same annulus have not, so far as I 

 have been able to ascertain, any direct communication with each other ; an indirect com- 

 munication, however, is perhaps established through the system of interseptal canals. But 

 each chamber normally communicates with two chambers of the annulus within it, and also 

 with two of that which surrounds it, by large passages (shown in horizontal section in 

 fig. 7, a b, a b, a b, and in vertical section in fig. b,f,f,f, and as seen in perspective view in 

 fig. 5, ff, (/), which traverse the annular septa ; of these passages there are generally two, and 

 occasionally three, one placed directly or obliquely above the other, for each pair of chambers 

 thus to be brought into communication. Thus at each extremity of the oblong chamber, 

 there are normally two passages on one plane (fig. 6,/;/) leading to two chambers (c, c , d, d') 

 in each of the annuli next internal or external to it ; but since to each of these chambers 

 there may be two or even three passages on different planes, the total number at each end 

 may be foui-, five, or six. But since, on the other hand, each of the 'interpolated' chambers 

 communicates with only one chamber in the annulus next internal to it, it will have but 

 a single set of passages in place of two. 



497. The shelly plates which enclose the chambered plane above and below, are formed of a 

 succession of superimposed lamellse (figs. 2, 5). These lamelte, which are of tolerably uniform 

 thickness, are most numerous in the older or more central portions of the disk, and diminish 

 in number towards the marginal or last-formed portions ; so that it seems pretty certain that 

 new laraellse must be added from time to time, as the disk is augmented by the formation of 

 new annuli. I have often met with appearances which might seem to indicate that the 

 formation of a new lamella over the entire surface of the disk, and the addition of a new 

 annulus at its margin, were parts of one and the same act of growth, the new lamella being 

 continued into the annular septum ; but if this were constantly the case, the number of 

 lamellre which form the ceiling or floor of any chamber would always correspond with the 

 number of annuli internal to it, which I do not find to hold good. Each of these ]amell?e is 

 perforated by an assemblage of parallel tubuli very closely set together, which pass from its 

 inner towards its outward surface (figs. 3, 4) ; and there is such a continuity between the 

 tubuh of successive lamellae, that a communication is thus established between the 

 cavity of the thickest-walled chamber and the external surface of the disk. These tubuli, 



