804 MICROSCOPIC FORMS OF ANIMAL LIFE 



any row normally communicates with two chamberlets in each of the 

 adjacent rows. The later turns of the spire very commonly grow com- 

 pletely 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. 606, in the illustration on the extreme right, 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 inclosed 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 the middle 

 illustration in the same figure. 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 pre- 

 sents 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 gencrnl 

 arrangement, such as will be more fully described hereafter in 

 Orbitolites. The largest existing examples of this type are far sur- 

 passed in size by those which make up a considerable part of a 

 Tertiary limestone on the Malabar coast of India, whose diameter 

 reaches seven or eight lines. 



A very curious modification of the same general plan is shown in 

 Alveolina, a genus of which the largest existing forms (fig. 608) are 

 commonly about one-third of an inch long, while far larger speci- 

 mens are found in the Tertiary limestones of Scinde. Here the 

 spire turns round a very elongate 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 that intervene between these are marked trans- 

 versely by lines which show the subdivision of the principal chambers 

 into * chamber-lets.' The chamberlets of each row are connected with 

 each other, as in the preceding type, by a continuous gallery ; and 

 they communicate with those of the next row by a series of multiple 

 pores in the principal septa, such as constitute the external orifices of 

 the last-formed series seen on its septal plane at a, a. 



The highest development of the cyclical plan of growth which 

 we have seen to be sometimes taken on by Orbiculina is found in 

 Orbitolites ; a type which, long known as a very abundant fossil in 

 the earlier Tertiaries of the Paris basin, has lately proved to be 

 scarcely less abundant in certain parts of the existing ocean. The 

 largest recent specimens of it, sometimes attaining the size of a 

 shilling, have hitherto been obtained only from the coast of New 

 Holland, the Fijian reefs, and various other parts of the Polynesian 

 Archipelago ; but discs of comparatively minute size and simpler 

 organisation are to be found in almost all foraminiferal sands and 

 dredgings from the shores of the warmer regions of the globe, being 



