GENUS ORBITOLITES VARIATIONS IN SURFACE-MARKINGS. 
215 
familiarised with their variety by the examination of a considerable number of speci- 
mens, they would become sources of great perplexity. We have already seen that 
the cellular markings present two very distinct forms, the rounded (Plate V. fig. 1) 
and the oblong (Plate V. fig. 6); 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 coexistence of both these plans of structure in a single individual (^ 33.), 
sufficiently proves that the diversity of the surface-markings to which they respect- 
ively 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 my description of 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. 
49. Although each surface, in either of the two principal types, ordinarily shows a 
division into concentric zones, which are again transversely subdivided so as to mark 
the separation of the cells, yet sometimes the concentric zones are alone visible, and 
no transverse subdivision is indicated, save by the alternation of lights and shadows 
proceeding from a like alternation of solid substance and of hollow spaces beneath 
(Plate VII. fig. 7). This predominance of the concentric divisions, which gives a 
very distinetive aspect to the disks which exhibit it, is usually most apparent in indi- 
viduals whose vertical section exhibits two planes of cells ; and it has seemed to me 
to depend on the 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 (on whose occurrence in fossil specimens I 
believe the genus Cyclolina to have been founded, ^ 5.) may pass into either of the 
principal types previously noticed; thus in fig. 14 we observe the concentric zones, 
though still very strongly marked, breaking up (so to speak) into bands of rounded 
cells with slightly convex covers ; whilst in figs. 5, 6 they are subdivided by very defi- 
nite transverse lines into cells of remarkable squareness, which still retain the original 
flatness of their surfaces. 
50. 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 (Plate VII. fig. 8), and the 
boundaries of the cells being formed by the intersection of these with each other. 
This aspect, however, which seems due to an unusual freedom in the oblique com- 
munications between the cells in each zone and those alternating with them in the 
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