GENUS ORBITOLITES. 121 



178. The appearances presented on minute observation by the surface-marUngs of Orhi- 

 tolifes, which indicate the form and arrangement of its contained chamberlets, are so far from 

 being uniform, that to any one whose eye had not become familiarised with their variety by 

 the examination of a considerable number of specimens, they would become sources of great 

 perplexity. We have already seen that the subdivisions of the annuli visible externally 

 present two very distinct forms, the rounded or oval (Plate IX, fig. 1), and the rectangular 

 (Plate IX, figs. 7, 9) ; the first of these being specially characteristic of that Simpler type of 

 structure in which there is only a single layer of cells, but not being confined to it ; whilst 

 the second is peculiar to the Complex type, in which there are two superficial layers distinct 

 from the intermediate stratum. Now the occasional co-existence of both these plans of struc- 

 ture in a single individual (t 172) sufficiently proves that the diversity of the surface 

 markings to which they respectively give rise, cannot be regarded as a basis for specific 

 distinction ; and when these extremes of diversity are kept in view, it must be felt to be 

 highly improbable that any modifications of either form should possess greater importance 

 That such modifications are mere individual varieties, is further evidenced by their gradational 

 character, and by the fact that two or more of them may present themselves in the same 

 disk. In describing them, I shall limit myself to an account of those more remarkable and 

 frequently-recurring varieties, which will serve, I think, as a key to any others that are 

 likely to be met with. — Although each surface, in either of the two principal types, ordi- 

 narily shows a division into concentric zones, which are again transversely subdivided so as 

 to mark the separation of the chaniberlets, yet sometimes the concentric zones are alone 

 visible, and no transverse subdivision is indicated, save by the alternation of hghts and 

 shadows proceeding from a like alternation of solid substance and of hollow spaces beneath. 

 This predominance of the concentric divisions, which gives a very distinctive aspect to the 

 disks that exhibit it, is usually most apparent in individuals whose vertical section exhibits 

 two planes of cells ; and it has seemed to me to depend on an unusual freedom between the 

 lateral communications, which I have noticed in certain individuals thus formed, so that the 

 animal portion of each zone might be described as an annulus of sarcode, merely constricted 

 at intervals. This peculiarly cyclical aspect of the surface may pass into either of the 

 principal types previously noticed ; the concentric zones sometimes breaking up (so to speak) 

 into bands of rounded chamberlets with slightly convex roofs ; whilst in other instances they 

 are subdivided by very definite transverse lines into cells of remarkable squareness, which 

 still retain the original flatness of their surfaces. — On the other hand, the appearance of 

 concentric division is sometimes almost entirely wanting ; the surface of the disk exhibiting 

 excentric circular markings, which resemble those of an engine-turned watch-case, and the 

 boundaries of the cells being formed by the intersection of these with each other. This 

 aspect, which seems due to an unusual freedom in the oblique communications between the 

 cells in each zone and those alternating with them in the contiguous zones on either side, 

 insensibly passes into the ordinary type ; and it is not uncommon to meet with disks, espe- 

 cially fossil, which exhibit in one part the engine-turned aspect, and in another that of 

 concentric zones transversely subdivided. Indeed I have sometimes found that the very 

 same disk might be made to present either of these aspects, according to the manner in 

 which the light is made to impinge upon it and is reflected from it. — Although the rounded 

 or oval form of the superficial divisions is specially characteristic of the Simple type of 



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