542 
DE. CAEPEXTEE’S EESEAECHES OX THE rOEA:MIXIFEEA. 
the earlier four or five of these completely iuvest the preceding, then chambers extend- 
ing on either side to the centre of the spire, as is partly sho-wn in the vertical section 
(Plate XVII. fig. 2 ) ; but as new whorls are added around these, the chambers cease to 
be thus prolonged over the preceding whorls, which would consequently be apparent 
externally if not concealed by the nucleus. The distance between the successive septa 
remains nearly the same after the spire has made two or three turns ; and thus the size 
of the segments, as seen in an equatorial section, remains pretty much the same through- 
out all the later growth of the shell, while the number of chambers in the successive 
convolutions increases nearly in proportion to the length of those convolutions. 
184. In these particulars, therefore, Polystomella corresponds rather with the Cyclo- 
stegue than with the ordinary Helicostegue Foraminifera; and this correspondence is 
further borne out by the existence of an obvious relation in the position of the chambers 
of successive whorls, which exists in the former, but which cannot be traced in those 
forms of the latter to which our attention has hitherto been directed. It has been 
shown that, in Orbitolites and Gycloclypeus^ the chambers of each concentric zone 
normally alternate in position with those of the zones which adjoin it internally and 
externally 17, 18, 100); and that this relation arises out of the mode of their 
communication with each other. In Peneroplis, Opercidina, and Amphistegina. on the 
other hand, the position of the chambers of each successive whorl appears to be per- 
fectly independent of that of the chambers in the whorl which preceded it. Xow in 
the Polystomella whose structure we are considering, such an alternating arrangement 
appears to be the normal one, as is shown in the relation of the chambers b. b, Plate XVII. 
fig. 8, to a, a, and c,c; so that lines drawn from the centre of the sphe through the septa 
of one convolution would pass through the middle of the chambers of the next, and would 
again meet the septa of the convolution beyond. This arrangement is shown in Plate 
XVII. fig. 7, as it presents itself in an actual section, and more diagrauimatically in 
Plate XVIII. fig. 1 ; in both the vertical plane of section lays open the chambers a. a\ 
whilst it traverses the septa b, b^, b'^ of the alternating whorls. It is, however, by no 
means constant; being very liable to be disturbed by that interpolation of additional 
chambers, which is required for the augmentation of their number in successive whorls. 
We shall presently see 189) that although this relation does not depend, as in Orbit o- 
lites and Cycloclypeus, upon direct communications between the chambers of successive 
rows, it is manifestly connected with the peculiar disposition of the canal-system, which 
here acquires a remarkable development and importance. 
185. Although, however, there is but little progressive increase in the dimensions of the 
successive chambers, and of the segments of the sarcode-body which occupy them, as seen 
in sections taken through the equatorial plane, it is made obrious by sections made at 
right angles to this (Plate XVII. fig. 2), that a rapid augmentation takes place m what 
may be termed the meridional direction ; the distance between the two lateral sui'faces 
of each whorl being considerably greater than between those of the preceding, so that 
the chambered portion of the shell progressively increases in thickness from the centre 
