220 DR. CARPENTER'S RESEARCHES ON THE FORAMINIFERA. 



plex type begins to be assumed, sufficiently explains the fact already noticed (^[ 22.), 

 that although the disks of the 'simple' type are for the most part of minute size, yet 

 that the diameter of many of them exceeds that of the smaller disks of the complex 



type. 



58. Even when an intermediate stratum is formed by the separation of the annular 

 canals, the superficial cells are not always clearly marked-off from its columnar 

 cavities ; for instead of being separated by floors formed by the expanded summits 

 of the zonal septa (^[ 28.), they sometimes open at once into the columnar cells of 

 the intermediate substance, so as to be quite continuous with them. Of this we 

 have an example in the three zones 6 8 of the specimen represented in Plate V. 

 fig. 10, b c. This continuity of the superficial cells with the intermediate columns, 

 is sometimes maintained throughout the disk, so that in no part of it are the former 

 clearly marked off from the latter ; as is seen in the portion b d of fig. 7, Plate V. ; 

 in which, however, the intermediate layer is much less regular than usual. This 

 method of growth is so remarkably constant in the Fossil Orbitolites of the Eocene 

 strata, whose intermediate layer is fully and very regularly developed (see Plate VI. 

 figs. 10, 11, and Plate VIII. fig. 2), that it might be considered to be specifically 

 characteristic of them, did we not occasionally find it to occur in certain zones of 

 recent disks, which are elsewhere exactly conformable to what I have described as the 

 regular type. Thus in the vertical section represented in Plate V. fig. 10, we see that 

 whilst the superficial cells of the three zones b c are continuous with the columns 

 of the intermediate stratum, a change then occurs in the relative places of their zonal 

 septa, so that the cells of the former come to be, as it were, detached from the 

 columns of the latter, and to have floors formed by the summits of the partitions by 

 which these are divided. It is, as already remarked (^j"51.), where the superficial 

 cells are continuous with the columnar cells of the intermediate substance, that they 

 present the rounded or ovoidal shape, instead of the elongated straight-sided figure 

 which is their characteristic form. And the former seems to give place to the latter, 

 whenever the cells of the superficial layers are perfectly separated from those of the 

 intermediate stratum, and are connected only with the annular passages. 



59. The intermediate stratum, again, may be altogether wanting, notwithstanding 

 that the two superficial layers are separated from each other by a horizontal partition. 

 In this case, each layer has its own annular canal ; and its cells have sometimes such 

 an arrangement as regards those of the other layer, that one of the connecting stolons 

 from which each segment arises, will pass into the alternating cell of the upper layer, 

 and the other into that of the lower. This arrangement may present itself as one of 

 the modes of transition from the simpler to the more complex type, as is shown in 

 Plate V. fig. 9; the columns being disposed to subdivide transversely when they 

 attain a considerable length, and the annular canal becoming double ; whilst in zones 

 more distant from the centre, the two layers are separated by the interposition of the 

 intermediate stratum. Sometimes, however, the disk continues to increase and attains 



