368 PROF. EHRENBERG ON ANIMALS OF THE CHALK 
EXPLANATION OF THE PLATES. 
The four Plates annexed to this Memoir, containing all the living 
microscopic marine animals that have not been previously figured, 
magnified 300 times in diameter, are moreover intended to afford three 
views of a more general nature. 
In the first place, they will give a more extended view of hitherto 
doubtful and unknown organic beings, of numerous living native Poly- 
thalamia, while the structural relations previously communicated in 
the Memoir on the formation of Chalk were taken from dead animals, 
and, with the exception of a single foreign species, only illustrated the 
dead structure, not the living form. 
Secondly, they will place in view now living forms of calcareous- 
shelled and siliceous-shelled animalcules, which can be regarded in no 
other light than as identical with those species that form in masses the 
chalk and chalk marl of the secondary formation of the earth. 
And thirdly, they will give an idea of a portion of the rich booty 
yielded by a single pail-full of sea water in microscopic and hitherto 
quite unknown forms, almost all of great importance in geological in- 
quiries. 
The two first Plates contain only calcareous-shelled animalcules, 
Polythalamia; the other two only siliceous-shelled animalcules, Jr- 
fusoria. 
The Plates were executed at a time when only thirteen living species 
of chalk animalcules were known, and contain therefore only these. 
PLATE V. 
The first and second Plates contain six species of native and recent 
caleareous-shelled animaleules belonging to the group Polythalamia, 
of which four species appear to be identical with those forming the 
mass of the chalk. 
The whole of the first Plate is occupied by the representation of 
Geoponus Stella borealis, as representative of the family of the Heli- 
cotrochina, in the living state, with its organic detail. It has been 
found alive only near Cuxhaven, and is not known from the chalk. 
The great similarity of this composite animal to the single-bodied 
animal of the Nonionina, represented on the second Plate, is highly 
remarkable. 
Fig. a. The creature magnified 290 times, with numerous projected 
very delicate locomotive organs whilst in motion, and in the greatest 
observed development and projection of the soft parts from the shell. 
At first view it will be seen that the calcareous shell of the animal- 
cule is no interior part, like the dorsal plate of the naked Sepia, but 
is an external envelope like a snail’s shell. From the character of the 
septa which divide the inner space of the shell into several chambers, 
results further an important distinction from all molluscous shells; 
but a great similarity with the many-chambered house of the Nautilus 
and similar shell-bearing Sepiade is evident. 
