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of spiral turns also varies considerably in different specimens, and 
evidently according to the size and growth of the body. Thus, in 
those of a quarter of an inch in diameter, the number of whirls is 
from three to four ; and in those of an inch in diameter, the number 
is not less than two dozen. One of these bodies being split trans- 
versely through the spiral, beginning in the centre, and continued to 
the circumference, is seen on each of the inner faces. These spiral 
turns are divided into numerous chambers, by transverse, supposed 
imperforated septa, which are a little obliquely extended towards the 
centre of each disk, and become gradually lost between the approxi- 
mated plates. An idea of this part of the structure is given Plate X. 
Fig. 14. In fact, the external plate of each turn of this spiral is folded 
in two, its particular chambers being formed in the angle of the fold, 
which then is extended above and below, covering and uniting itself 
with the outer sides of the inner plates. The specimen, Plate X. 
Fig. 15, will point out this curious mode of structure. Little processes 
may be seen going off from the different fractured edges of the plates 
in the centre, to form the septa in their correspondent whirls. 
The same specimen will illustrate a part of the structure which does 
not appear to have been yet explained. On the surface of the middle 
plate of this specimen, small undulating risings are observable : these, 
by their attachments to the under side of the plate which laid over 
this, connected the two plates together, leaving a vacant space between 
the plates, where these risings did not exist. As these seeming irre- 
gularly undulating lines approach the circumference of the plate, 
they assume a greater regularity in their direction, and give off, at 
equal distances, the septa dividing the chambers which are disposed 
round the folded edge of the plates. 
In another species, Plate X. Fig. 17, this connection between the 
upper and under side of the plates is formed by numerous small and 
short columnar projections, the points of attachment of which may 
be seen on both surfaces. 
