GEXTJS POLTSTOI^IELLA INTEENAL STEUCTUEE. 
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 Ijdng 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 Williamsox, and as I have shown to be the case also in the higher types 
of Foraminifera generally. But fui’ther, 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 sui’face, corresponding to that “ intermediate skeleton,” of 
which less developed examples have already been furnished by Cycloclypeus, Hetero- 
stegina, Operciilina^ and AmpMstegina^ — 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 Orhitolites (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 Ileterostegma, Peneroplis, Operculina, and AmpM- 
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 : 
