GENUS ORBITOLITES. 117 



teristic development ; and the differences between this and the Simple type previously 

 described are such as at first sight to preclude the idea of their specific identity. But when 

 a large number of specimens are carefully examined and compared with each other, it 

 becomes obvious that not only may a vast amount of diversity present itself in the arrange- 

 ment of the chamberlets of the shell and of the segments of the animal,— so that one after 

 another of the characters which at first seem most clearly marked and therefore most 

 distinctive, may be shaded off" (so to speak) in such a manner as to establish a complete 

 transition between the two types, — but they frequently coexist in the very same disk. Such 

 a ^coexistence is exhibited in the vertical section represented in Plate IX, fig. 8, where we 

 see that the zones b — k, which immediately surround the "primitive disk," are formed in all 

 respects upon the Simple type ; between k and « we see an incipient diff'erentiation between 

 the " superficial " and the "intermediate" strata, the annular canal, however, still remain- 

 ing single ; but at n, the annular canal becomes double, and from this point to the margin 

 of the disk we see the " superficial " layers completely diff"erentiated from the " intermediate," 

 and the former becoming more and more widely separated from each other by the increasing 

 thickness of the latter. Now as the Complex type of growth may show itself in the very 

 first annular zone, or may thus evolve itself out of the Simple at any distance from the 

 centre, and this either suddenly or gradually, it seems obvious that between these two types 

 no essential distinction can exist. If the growth of such a disk as that whose vertical 

 section is represented in fig. S, had been stopped at k, or even at n, it would have 

 undoubtedly been regarded as belonging to the Simple type ; and every disk formed upon 

 the Simple type must be considered as having the power to evolve itself upon the Complex 

 plan. It would not be right, however, to afiirm that the Simple disks are the young of the 

 Complex, since we find that the former may continue to increase without change, until 

 they far exceed in diameter and in number of zones the smaller disks which have early 

 assumed the latter type. Of the conditions which determine the original evolution of the 

 Simple or of the Complex type respectively, from "primitive disks " which appear to be in 

 all respects identical, or which determine the evolution of the latter from the former, we 

 know nothing whatever. 



173. Even where the annular canals have been separated from each other, and a 

 distinct " intermediate stratum " has been formed between them, the superficial chamberlets 

 are not always clearly marked off" from its cylindrical cavities ; for instead of being separated 

 by floors formed by the expanded summits of the zonal septa (^ 166), they sometimes open 

 at once into the chamberlets of the intermediate layer, so as to be quite' continuous with 

 them ; and this continuity of the superficial with the intermediate chamberlets is sometimes 

 maintained throughout the disk, so that in no part of it are the former clearly marked off" 

 from the latter. 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, 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 J 

 have described as the regular type. Where the superficial chamberlets are continuous with 

 those of the intermediate substance, they present the rounded or ovoidal shape, instead of 

 the elongated straight-sided figure which is their characteristic form ; and the former seems 



