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 distinctive 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 rne 

 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 CycloUna 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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