June, 1891 . 
ANIMAL PEDIGREES. 
123 
communicates freely with that of the neighbouring chambers ; 
and through the marginal apertures at the rim of the shell 
pseudopodia can be protruded and food captured and ingested. 
An Orbitolite grows by addition of new chambers round 
the margin of the shell. The protoplasm, becoming too 
abundant to be contained within the cavities of the shell, 
protrudes as a rim all round the margin of the shell, and 
by deposition of calcareous matter gives rise to a new 
chamber to the shell. 
The discoidal shape of shell, so characteristic of Orbito- 
lites, is very unusual amongst Forammifera, and it is a 
matter of great interest to determine in what way it has 
been acquired. Bearing in mind what has just been said as 
to the mode of growth of the shell, it is clear that the oldest 
part, i.e., that which alone was present in the young animal, 
is the central portion, and that the successive concentric 
rings are younger and younger as we pass outwards towards 
the circumference, the marginal chamber being the youngest 
and latest formed of the whole series. Now, if we look at 
the central or oldest part of an Orbitolite, we find, Fig. 4, that 
it has not the concentric arrangement of the peripheral 
part, but is coiled spirally like the majority of Foraminifera. 
The central, or oldest, turns of this spiral are not chambered; 
the outer turns are divided by partitions into chambers, and 
these chambers, as we follow the spiral round, become 
wider and wider, so as to overlap and wrap round the older 
part of the shell, at first partially, but ultimately completely ; 
the first chamber that completely surrounds the shell 
marking the transition from the spiral to the discoidal type. 
We thus find that the discoidal Orbitolite shell commences 
its development as a spiral shell, and acquires the discoidal 
character merely through an exaggerated mode of growth on 
the part of the spiral. The Recapitulation Theory tells us that 
this is to be interpreted as meaning that the discoidal shells 
are descended from spiral ancestors, and the close agree¬ 
ment between a young Orbitolite, Fig. 2, and an adult 
Peneroplis, Fig. 1, suggests that either Peneroplis itself, or 
forms closely allied to it, were the actual ancestors. 
The Orbitolite is peculiarly instructive, owing to the 
fact that the addition of new chambers during growth takes 
place in such a way as to leave the older parts of the shell 
unaltered, and fully exposed to view, so that simple inspec¬ 
tion of an adult shell, as in Fig. 4, reveals the whole course 
of development, and shows us not merely the anatomy but 
the embryology as well. It is as though a kitten were to 
develop into a cat, not by interstitial growth in all its parts, 
but by the addition of successive lengths to its nose, its ears, 
