GENUS ORBITOLITES -.GENERAL PLAN; COMPLEX TYPE. . 203 



27. The disks of this complex type are not distinguished from those of the simple 

 type already described, by any difference in the structure of the Nucleus ; and there 

 is frequently nothing specially characteristic in the structure of the zones that imme- 

 diately surround it. Each of the peripheral zones, however, consists of two super- 

 ficial layers, an upper and a lower, and of an intermediate stratum ; these will now 



be described seriatim. 



28. The superficial layers are formed of the (usually) oblong cells, whose contour 

 is indicated by the surface-markings ; when they are laid open horizontally, by rub- 

 bing away the thin shell which covers them in (Plate VI. fig. 3), it is seen that the 

 floor of each cell has an aperture at either end ; but no communication can be 

 traced, either through the side-walls between the contiguous cells of the same zone, 

 or through the end-walls, between the cells of successive zones. Moreover, there is 

 no such alternating arrangement of the cells of successive zones, as we have seen to 

 prevail in the simpler type (^[ 17); and they altogether seem to be quite independent 

 one of another. When this superficial layer is examined in a vertical section having 

 a radial direction (Plate VI. fig. 7), it is seen that the floors of its cells (a, a) are 

 formed by the expanded summits (d, d', d") of the irregular septa, which separate 

 from each other the columnar cells of the intermediate stratum (cccc); and that 

 the apertures at the two ends of the floor are the entrances to passages (e, e', e"), 

 which lead obliquely downwards (the passages on either side of the partition between 

 two successive cells of the superficial layer inclining towards each other) towards 

 these cavities. It is observable, moreover, that just at the point at which the con- 

 tiguous passages meet each other, there is always a round aperture (f,f,f) in the 

 partition (g, g) which divides the contiguous cells of each zone ; and when, in a hori- 

 zontal section, the superficial cells have been entirely ground away, so as to lay open 

 the most superficial part of the intermediate stratum, this part is found to be traversed 

 in each zone by a continuous circular canal (Plate VIII. fig. 3), with large rounded 

 openings that lead into the columnar cells beneath. The meaning of this arrange- 

 ment becomes obvious, when we examine the disposition of the animal substance 

 which occupies these cavities ; for we find, as might have been anticipated, that the 

 superficial cells are filled with segments of sarcode of corresponding shape (Plate IV. 

 figs. 4, 7, aa) ; and that whilst these have no direct connexion with one another, 

 each of them is connected by means of fleshy peduncles with the annular stolons 

 bb that run along its extremities ; whilst from the under side of these annular stolons 

 (fig. 4) descend the thick columns of sarcode (cc, c'c 1 ), which occupy the columnar 

 cells of the intermediate stratum. The absence of any essential dependence of the 

 segments of the superficial, and of those of the intermediate strata upon each other, 

 seems indicated by the fact that there is no constant numerical relation between 

 them, a circumstance which extremely perplexed me, until I had ascertained, by 

 examination of the animal, that the passages (Plate VI. fig. 7, e, e 1 , e") debouch, not 

 (as I had at first supposed) into the columnar cavities, but into the annular 



