ction of the tail, if it does not go so far as actually to
displace and move forward the internal organs, at least fits in well
with an arrangement in which the organs are so disposed" (p. 99).

The first memoir deals with the homologies of the opercular bones.
Geoffroy considers that the external opening of the ear corresponds to
the external opening of the gill-chamber, which lies between the
operculum and the pectoral girdle. The ear communicates with the buccal
cavity by the Eustachian tube, so does the branchial chamber by means of
the gill-slits. The auditory chamber of higher Vertebrates is,
therefore, the homologue of the branchial chamber in fish; the opercular
bones in fish and the ossicles of the ear in other Vertebrates stand in
close relation to this chamber; therefore the opercular bones are the
homologues of the ossicles of the ear, the interoperculum corresponding
to the malleus, the suboperculum to the lenticular, the minute lower
part of the suboperculum to the incus, the operculum to the stapes, and
the pre-operculum to the tympanic ring. In making these particular
determinations Geoffroy professes to be led by his principle of
connections. The pre-operculum has, he says, the same connections with
neighbouring bones as the tympanic bone in other Vertebrates, and the
other pieces of the gill-cover are homologised with particular
ear-ossicles according to the order in which they stand to one another.
The second memoir in the book deals with the sternum, and affords a very
good example of Geoffroy's method of dealing with the facts of
structure. We shall omit here any detailed reference to the other three
memoirs, which deal with the hyoid, with the branchial arches and the
structures which correspond in air-breathing Vertebrates, and with the
bones of the shoulder-girdle.

In the memoir on the sternum Geoffroy's first care is to arrive at a
definition of what a sternum is. He defines it partly by its functions,
partly by its connections, as the system of bones which covers and
protects the thorax, and gives attachment to certain groups of muscles.

The most highly developed sternum (according to this definition) is the
plastron of the tortoise, whose structure it dominates (p. 103). It is
important, therefore, to determine of how many bones the plastron is
composed, since the full number of elementary parts of which an organ is
composed is best seen when the organ is at the maximum of its
development.