GENUS POLYSTOMELLA. 281 



arranged segments of PoJi/sfomeUa strenglhencd and consolidated by a secondary calcareous 

 deposit upon their external surface, which constitutes a " supplemental skeleton" (^ 63). 



475. The dimensions of the central chamber in which the spire of P. craliculata commences 

 are extremely variable ; the difference between the extremes of its size being, in fact, not less 

 remarkable than that which I have shown to present itself in Orhitolites (^ 177). Tims in 

 one specimen we may trace a progressive diminution in the size of the chambers as we 

 approach the central chamber, which is itself no larger than the chambers in nearest proximity 

 to it ; whilst in another not only is the size of the earlier wliorls and of their component 

 chambers considerably greater than usual, but the central cell alone occupies about tlic 

 same space as the first 2J whorls of the preceding. The average seems to be intermediate 

 between these two extremes. The lu'cadth of the successive whorls increases much more 

 gradually than in most other nautiloid Foraminifera, in this respect resembling Xiimmtdinn ,- 

 and there is no tendency whatever, even in the oldest and most developed specimens, to that 

 rapid opening-out of the spire, which is so marked a feature of the older specimens of Pciie- 

 roplis, OpercuJina, and Heterostet/ina. The largest number of whorls I have met witli in any 

 individual is eleven : the earlier four or five of these completely invest the preceding, tlieir 

 chambers extending on either side to the centre of the spire, as is partly shown in the 

 vertical section (Plate XVI, fig. 3) ; 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 convo- 

 lutions increases nearly in proportion to the length of their gyration. The chambers of each 

 whorl seem normally to alternate in position with those of the adjacent whorls ; 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 tiie convolution 

 beyond. This arrangement, however, is by no means constant, being very liable to be 

 disturbed by that hderpohdloii of additional chambers which is required for the augmentation 

 of their number in successive whorls : it will presently be seen to be related to the peculiar 

 disposition of the canal-system (^f 479), which here acquires a remarkable development and 

 importance. 



476. 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 XVI, fig. 3) that a rapid augmentation takes place in what may be 

 termed the meridional direction ; the distance between the two lateral surfaces of eacli 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 towards the circum- 

 ference. The conical hollow thus left on each side in the central portion of the shell, is 

 entirely filled up by the solid nucleus already adverted to : the calcareous deposit of which 

 the nucleus is composed, however, is by no means limited to it, but extends over the whole 



36 



