550 DR. CARPENTER'S RESEARCHES ON THE FORAMINIFERA. 



same as that which is presented by those aberrant forms of Orbltolites, in which the 

 central cell buds-out only on one side, instead of all around (see ^[ 54, and Plate IX. 

 fig. 4). The spiral mode of increase is usually carried-on much further in Orbiculina, 

 than it ever is in Orbitolites ; several turns being made before it gives place to the 

 cyclical plan. But as this is a mere question of degree, such a difference would 

 scarcely alone afford a valid distinction between these two types. There is, however, 

 this very definite positive distinction, that in Orbiculina each turn of the spire not 

 only surrounds the preceding, but completely invests it above and beneath, every 

 band of new chambers being continued to the very centre ; so that, whilst the spiral 

 mode of growth continues, the thickness of the shell increases with each turn ; and 

 after this has given place to the cyclical, the central nucleus, thus augmented in 

 thickness, projects above the plane of the disk. This peculiarity in Orbiculina, which 

 I have never found to be wanting, and to which there is never the least approach in 

 Orbitolites, is best seen in vertical sections. Thus in Plate XXIX. fig. 3 is shown 

 that part of a vertical section of a large disk, which has passed through its nucleus; 

 the innermost and therefore first-formed portion of which is seen to be invested above 

 and below by three layer.*, formed by three turns of the spiral. The same peculiarity 

 is shown in Plate XXVIII. fig. 17, which represents a portion of a fossil disk near the 

 nucleus. 



86. The transition from the spiral to the cyclical mode of growth is effected in 

 Orbiculina, exactly as in the aberrant forms of Orbitolites just referred-to, by the 

 opening-out (so to speak) of the mouth of the spire; the successive rows extending 

 themselves more and more on either side, until they meet around the previously- 

 formed portion ; after which each new row forms a complete zone or annulus. The 

 commencement of this change is seen in Plate XXVIII. fig. 1 ; and its subsequent 

 stages are shown in figs. 2-5, a comparison of which will show that the specimens 

 which they represent are in different phases of this transitional state. 



87. But this transition by no means constantly occurs; for the original spiral plan 

 of growth is not (infrequently maintained, apparently through the whole of life; so 

 that specimens are often met-with, which are not inferior in size or in number of 

 rows to the larger disks, but which retain the aduncal form. Such a series of speci- 

 mens is shown in figs. 810; the first of which represents a very young Orbiculina, in 

 that stage which is common to both types of growth ; whilst it is obvious from a com- 

 parison of this with the two following, that their increase has continued to take place 

 upon the same plan, each row that is put-forth from its predecessor terminating 

 abruptly like it at its free extremity, without any such disposition to extend itself 

 as would carry it round the nucleus so as to form a complete annulus. A horizontal 

 section of such a specimen as is represented in fig. 10, is shown in fig. 12. It may, 

 of course, be urged that such a difference ought to be accounted sufficient to sepa- 

 rate the spiral and the discoidal types of Orbiculina, as two distinct species ; but the 

 following reasons appear to me quite sufficient to negative such a mode of viewing 



