542 DR. CARPENTER'S RESEARCHES ON THE FORAMINIFERA. 



the earlier four or five of these completely invest the preceding, their chambers extend- 

 ing on either side to the centre of the spire, as is partly shown 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 Helicosttyue 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 OrUtolites and Cycloclypeus, 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, Operculina, 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. Now 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 spire 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 diagrammatically in 

 Plate XVIII. fig. 1 ; in both the vertical plane of section lays open the chambers a, a\ a?, 

 whilst it traverses the septa b, b\ b 3 , b 3 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 OrUto- 

 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 obvious by sections made at 

 right angles to this (Plate XVII. fig. 2), that a rapid augmentation takes place in what 

 may be termed the meridional direction ; the distance between the two lateral surfaces 

 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 



