miliolida; orbiculina; alveolina. 401 



namely, which has received the designation Orbiculina (Plate xy., 

 figs. 6, 7, 8). The relation of this to the preceding will be best 

 understood by an examination of its early stage of growth, repre- 

 sented in fig. 7 ; for here we see that the Shell resembles that of 

 Peneroplis in its general form, but that its principal chambers are 

 divided by Secondary Septa passing at right angles to the primary, 

 into ' chamberlets ' occupied by sub-segments of the sarcode-body. 

 Each of these secondary septa is perforated by an aperture, so that 

 a continuous gallery is formed, through which there passes a stolon 

 that unites together all the sub-segments of each row. The Cham- 

 berlets of successive rows alternate with one another in position ; 

 and the pores of the principal septa are so disposed, that each 

 chamberlet of any row normally communicates with two chamber- 

 lets in each of the adjacent rows. The later turns of the Spire 

 very commonly grow completely over the earlier, and thus the 

 central portion or ' Umbilicus ' comes to be protuberant, whilst the 

 growing edge is thin. The Spire also opens-out at its growing 

 margin, which tends to encircle the first-formed portion, and thus 

 gives rise to the peculiar shape represented in fig. 8, which is the 

 common aduncal type of this organism. But sometimes, even at 

 an early age, the growing margin extends so far round on each 

 side, that its two extremities meet on the opposite side of the 

 original spire, which is thus completely enclosed by it ; and its 

 subsequent growth is no longer spiral but cyclical, a succession of 

 concentric rings being added, one around the other, as shown 

 in fig. 6. This change is extremely curious, as demonstrating the 

 intimate relationship between the spiral and the cyclical plans of 

 growth, which at first sight appear essentially distinct. In all 

 but the youngest examples of Orbiculina, the septal plane presents 

 more than a single row of pores, the number of rows increasing in 

 the thickest specimens to six or eight. This increase is associated 

 with a change in the form of the sub-segments of sarcode from 

 little blocks to columns, and with a greater complexity in the 

 general arrangement, such as will be more fully described in a sub- 

 sequent paragraph (§ 379). The largest existing examples of this 

 type are far surpassed in size by those which make up a consider- 

 able part of a Tertiary Limestone on the Malabar coast of India, 

 whose diameter reaches 7 or 8 lines. ' 



375. A very curious modification of the same general plan is 

 shown in Alveolina, a Genus of which the largest existing forms 

 (Fig. 249) do not attain the size of the smallest sugar-plum, but 

 of which we find specimens in the Tertiary Limestones of Scinde 

 not less than three inches in length and more than an inch in 

 diameter. Here the Spire turns round a very elongated axis, so 

 that the Shell has almost the form of a cylinder drawn to a point 

 at each extremity. Its surface shows a series of longitudinal lines 

 which mark the principal Septa ; and the bands which intervene 

 between these are marked transversely by lines which show the 



