There are nine bones in the plastron of the tortoise. "The
conclusion to be drawn from this is that every sternum, provided that it
is not inhibited in its development by some obstacle, is composed of
_nine elementary parts_" (p. 105). These nine bones are in Geoffroy's
nomenclature, the episternals, the hyosternals, the hyposternals, the
xiphisternals, which are all paired bones, and the entosternal, which is
unpaired. The arrangement of them is in the tortoise:--

Episternal---------------------------Episternal
|\__ __/|
| \__ __/ |
| \__ __/ |
| \__ Entosternal __/ |
| __/ \__ |
| __/ \__ |
| __/ \__ |
|/ \|
Hyosternal Hyosternal
| |
| |
| |
| |
Hyposternal-------------------------Hyposternal
| |
| |
| |
| |
Xiphisternal------------------------Xiphisternal.

The articulations in the tortoise are indicated by the connecting
lines. Geoffroy tries to show that the sternum in other animals is
composed of these nine bones, or at least of a certain number of them,
always in the same invariable relative positions. Thus in birds the
sternum consists of five pieces, of a huge keeled entosternal, and of
two "annexes" on either side, which are the hyo-and hyposternals.
These are separate only in young birds. Occasionally, especially in
young birds, rudiments of episternals and xiphisternals also occur.
The minuteness of the episternals and the xiphisternals may be
attributed to the gigantic size of the entosternal, in accordance with
the _Loi de balancement_. In the other air-breathing Vertebrates the
nine sternal elements can according to Geoffroy be discovered without
great difficulty. But when we come to the determination of the sternum
in fishes, difficulties abound, which Geoffroy solves in the following
way. He points out that between the clavicles (_cleithra_) and the
hyoid bone (_basihyal_) in fishes there is a long median bo