180 _  GEOGNOSY AND GEOLOGY. 
while it negatives the cephalopod character, renders it probable that the 
shell formed a minute polypidom, inhabited by animals of a very low 
organization. The whole of the true chalk appears to have been formed 
from such shells) We need not be astonished at such abundance of 
infusorial shields when we reflect that one animal whose calcareous or 
silicious test may weigh ;;ta0th of a grain, is capable of so multiplying 
itself in thirty days. as that the sum of the resulting tests or shields shall 
weigh over 65,000,000,000 Ibs., and be capable of covering a surface nearly 
fifteen square miles to a depth of two feet, with a density equal to that of 
water. The Rhizopods are divisible into various groups, according to the 
various position and character of the individual chambers. The Monostegia 
are shells with a single chamber; the Stichostegia have the chambers 
placed one above the other in a single straight or curved line; the 
Helicostegia, with chambers placed along an axis forming a spiral volute ; 
the Entomostegia, with the chambers placed along two axes alternating with 
each other and rolled up into a spiral; the Hnallostegia, with the chambers 
alternating along two or three axes, and not rolled up spirally: and the 
Agathistegia, or milliolites, in which the chambers are disposed spirally 
round an axis, each one occupying a semi-circumference. 
Dentalina, which belongs to the Stichostegia, has a somewhat curved 
conical shell, consisting of spherical chambers, the latter often elongated. 
The single cells are separated from each other by no very deep contractions : 
Dentalina sulcata (pl. 39, fig. 3). 
Textularia, of the order Enallostegia, has a conical and regular shell, with 
the round or wedge-shaped chambers arranged along two contiguous axes : 
T. aciculata ( fig. 4). 
The Helicostegia separate into two groups, according as the chambers 
are rolled up in a plane or in a turret. Bulimina belongs to the latter: B. 
sbliqua ( fig. 5). Between these and those whose turns lie in one plane, 
ike the nautilus, stand the Rotalina, whose obliquely rolled columella is very 
short: R. voltziana (fig. 6). The Cristellaria exhibit the greatest 
resemblance to the nautilus, being a perfect miniature of the latter: C. 
rotulata (pl. 39, fig. 7). As Lituites is only a modified nautilus form, so 
Lituola may be considered as such with regard to Cristellaria. They are 
cristellaria whose cells suddenly lose the winding character, and run out in 
a straight direction: L. nautiloidea (fig. 8). The flat compressed shell of 
Flabellina is at first wound up very regularly, and afterwards expands in a 
foliated manner. The partition walls are at first simply curved, and 
afterwards interrupted: F. rugosa (fig. 9). 
The infusoria hitherto considered are those found in the white chalk. In 
other calcareous masses of the cretaceous system allied forms occur, as in 
the nummulite limestone which occurs in the Pyrenees. The nummulites 
fill the above-mentioned series in countless numbers, and would be highly 
characteristic did they not also occur in the Hippurite limestone. The 
internal structure of the nummulite is often beautifully revealed on breaking 
open a piece of nummulite limestone, in which case it will most frequently 
happen that one or more of the shells will be split in two, as they are of 
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