GENTJS POLYSTOMELLA : INTERNAL STKTJCTUEE. 541 



differs remarkably from most of the other nautiloid Foraminifera ; the breadth of each 

 of the later whorls being many times exceeded by what may be termed its thickness, that 

 is, by the distance between its two lateral surfaces. Thus the segments come to have 

 somewhat of the form and arrangement which the carpels of an orange would exhibit, 

 if, instead of lying in a single circle round a central axis, they were disposed in a 

 succession of whorls, with a progressive increase in their dimensions. This comparison 

 may be conveniently carried a little further. For as each carpel of the orange has its 

 own investing membrane, so that the partitions between the adjacent carpels are double, 

 so each segment of Polystomella has its own proper shelly investment, causing the septa 

 which separate the adjacent segments to be double, as was originally pointed out by 

 Professor WILLIAMSON, and as I have shown to be the case also in the higher types 

 of Foraminifera generally. But further, as the separate carpels of the orange are 

 collectively invested by a general integument, which also to a certain degree dips down 

 between them, and which fills up what would otherwise be void spaces about the two 

 poles of the spheroid, so shall we find that the proper walls of the spirally arranged 

 segments of Polystomella are strengthened and consolidated by a secondary calcareous 

 deposit upon their external surface, corresponding to that " intermediate skeleton," of 

 which less developed examples have already been furnished by Cycloclypeus, Hetero- 

 stegina, Operculina, and Amphistegina, its most distinctive peculiarity in Polystomella 

 being its extraordinary thickness on the two lateral surfaces of all but the last formed 

 whorl. 



183. The spire of Polystomella, like that of other nautiloid Foraminifera, commences 

 in a central cell, the dimensions of which 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 Orbitolites (First Series, ^[ 44). Thus in Plate XVII. fig. 3, 

 which represents a section of the five inner whorls of a full-grown specimen, taken 

 through the equatorial plane, we trace a progressive diminution in the size of the 

 chambers as we approach the central cell, which is itself no larger than the chambers 

 in nearest proximity to it. In fig. 4, on the other hand, which represents a correspond- 

 ing section of the inner portion of another specimen, drawn under the same magnifying 

 power, we see that not only is the size of the earlier whorls and of their component 

 chambers considerably greater, but that the central cell alone occupies about the same 

 space as the first 2^ whorls of the specimen represented in fig. 3. The average seems 

 to be intermediate between these two extremes. The breadth of the successive whorls 

 increases much more gradually than in most other nautiloid Foraminifera, in this 

 respect resembling Nummulites rather than the recent forms described in former 

 memoirs; and there is no tendency whatever, even in the oldest and most developed 

 specimens, to that rapid opening-out of the spire, which we have seen to be so marked 

 a feature of the older specimens of Heterostegina, Peneroplis, Operculina, and Amphi- 

 stegina. The largest number of whorls I have met with in any individual (that, namely, 

 to be counted in the specimen whose inner portion is represented in fig. 3) is eleven : 



