GENUS ORBITOLITES. 107 



156. Simple Type : External Characters. — In sands and dredgings brought from the seas 

 of any of the warmer regions of the globe, and especially in those from the Philippine shores, 

 we almost constantly meet with an abundance of minute Orbitolite-disks, whose diameter is 

 ordinarily about '05 of an inch, and which usually contain from ten to fifteen concentric zones. 

 These constitute the OrhiloKtes marginalis of Lamarck (lx), the Sorites orbicuhs oi Ehren- 

 berg (xxxix). Although these disks are typically plane or nearly so (there being usually 

 no great difference between the thickness of their central and that of their peripheral region), 

 yet it not unfrequently happens that the successive zones gradually increase in thickness 

 from within outwards (as is shown in Plate IX, fig. 2), so that the disk becomes somewhat 

 biconcave. Sometimes, again, without any alteration in the thickness of the several parts, 

 the disk comes to assume, by the depression of its central portion, the shape of a plate, or 

 that of a watch-glass, or (by the more complete upturning of its edges) that of a saucer. 

 In any case in which either surface of the marginal zone is more exposed by its projection 

 than those of the zones which it encloses, there will be a special liability to a laying-open of 

 its chamberlets if the disk should be subjected to attrition ; and I doubt not that in this 

 mode have been produced the supposed large pores of the fossil 0. macropora, the figure of 

 which given by Goldfuss (' Petrefacta,' pi. xii, fig. 8) corresponds exactly with recent specimens 

 I have frequently encountered, presenting a row of large ovate apertures along the surface 

 of the outermost zone. The true pores (which have been very commonly overlooked) are 

 situated in the depressions between the marginal convexities (Plate IX, fig. 1,/,/) ; in each 

 of these depressions there is but a single pore, and this is almost always of a regularly 

 circular form, and is surrounded by a prominent annulus of shell. 



157. Internal Structure.— When the interior of an Orbitolitcs of simple type is laid open 

 by a section that passes in a direction parallel to one of its surfaces (which in the case of 

 these discoidal forms will be most conveniently termed a horizontal section), as shown in 

 Fig. XXIV, the central portion is seen to be occupied by a large cavity, that is somewhat 

 irregularly divided by a sinuous partition ; notwithstanding some irregularity, this partition 

 always marks out a " primordial chamber," a, of a somewhat pyriform shape, from the 

 " circumambient chamber" b, b, which passes round it ; in this partition there is an aper- 

 ture, d, which establishes a communication between the narrow prolongation of the " pri- 

 mordial" or " central" chamber and the large " circumambient chamber." (See also Plate IX, 

 fig. 1.) The meaning of this arrangement will at once be made apparent by reference to 

 the disposition of the " primordial" and " circumambient" segments of sarcode, which occupy 

 the cavity of what may be conveniently termed the " primitive disk" (^ 161). In a vertical 

 section passing through the centre of this disk, such as that seen in Plate IX, fig. 2, it seems 

 to present three chambers ; but this is simply due to the fact that such a section will traverse 

 the circumambient chamber twice, that is, will cut it through on both sides {b, b) of the 

 central chamber a. Some sections present only two such chambers ; in this case the plane 

 of division seems to have traversed the "primitive disk" just where the neck of the 

 primordial chamber touches its margin, so that the circumambient chamber is only on 

 one side of it. If, on the other hand, the plane of division should happen not to pass 

 through the primordial chamber at all, so as to traverse the circumambient chamber 

 alone, a single broad cavity will present itself in the vertical section, as shown in fig. 8, b. 



