116 FAMILY MILIOLIDA. 



the marginal pores of any one vertical row, even when in a line with each other, open alter- 

 nately into the chamberlets on the rifjld and on the left of that row ; these pores being 

 nothing else than the orifices of the oblique inter-zonular passages, which, when another 

 annulus is added, will lead into its chamberlets. The import of this arrangement is at once 

 made evident by an examination of the segments of the animal body that occupy the cylindrical 

 chamberlets ; for, as is shown in Plate IV, fig. 25, each column of sarcode in one zone (c c) 

 communicates with the two columns alternating with it in the next zone {c'c) by two rows 

 of peduncles ; and the peduncles which pass from each pair of contiguous columns to the 

 single column of the next zone, incline towards one another, so as to enter it nearly in the 

 same vertical line, though in different horizontal planes. 



] 69. That which has been already stated in regard to the partial deficiency of the inner 

 wall in each of the concentric zones of the Simple type (^ 159), holds good also in regard 

 to the septa which divide the successive zones of the "intermediate stratum" in this more 

 Complex type ; for the walls of the cylindrical chamberlets close-in around them very imper- 

 fectly on their inner or central side, leaving large irregular vertical fissilres (Plate IX, fig. 11) 

 which are applied to the vertical rows of orifices (fig. 10) on the outer margin of the included 

 zone. 



170. The thickness of this "intermediate stratum," and the number of superposed 

 segments (indicated by that of the inter-zonular peduncles) of which each column of sarcode 

 consists, are found to vary considerably in different parts of the same disk ; being usually 

 least near its centre, and gradually augmenting in successive zones as their distance from 

 this increases (Plate IX, fig. 8) ; or ceasing to augment at a certain point, so that the outer 

 part of the disk is flat ; or even diminishing again, so that the disk thins away towards its 

 margin. It is specially worthy of note that whatever differences of this kind may exist, they 

 are entirely due to the variable length of the columns of the intermediate stratum ; the depth 

 of the chamberlets of the si'jjerficial layers being nearly constant, and no vertical multiplica- 

 tion of these ever taking place. 



171. The addition of new zones usually takes place Avith the same regularity in the 

 Complex as in the Simple type of structure ; but departures from this regularity, occasioned 

 by a want of completeness of particular zones, are more frequent ; and this is perhaps to be 

 accounted for by the larger size of the disk, which will tend to produce a less intimate 

 dependence of each part of the animal body upon every other, and will thus favour the 

 partial action of any cause {e.g. an excess of nutrient materials) which promotes a more 

 rapid growth on one side than on the other. And this view is most remarkably borne out 

 by the fact, that in the genus Cychchjpeus, which, though normally growing upon the cyclical 

 plan, possesses a much greater degree of segmental independence, such irregularities occur 

 far more frequently ; so that, in fact, it is rare in that type to meet with a disk whose increase 

 has taken place with uniformity throughout. 



172. The foregoing description apphes in every particular to those specimens only, 

 which present the structure of the Complex type of Orbitolifes in its most regular and charac- 



