114

92

The four ribless sacral vertebrae, which in the Dinornis are interposed between those
which send their anchylosed ribs to abut upon the os innominatum anterior to the ace-
tabulum, and those which strengthen in like manner the posterior part of the acetabu-
lum, are very short ; their bodies have coalesced into a single mass of bone, smooth and
flattened below, rounded at the sides, and only recognizable as distinct bones by the
orifices for the nerves at the sides of the anchylosed mass : these orifices are double,
as in the sacrum of other birds 1, the two roots of the nerves escaping separately, the
motor root issuing by the lower, the sensitive root by the upper orifice.

The upper transverse process of the first sacral vertebra is a broad and thick piece
of bone, extending from the body and anterior articular process of the vertebra, and
having a deep and smooth excavation at its anterior part : in the Ostrich the correspond-
ing part is much smaller and is reticulated by the bars of bone dividing the orifices by
which the air is admitted into the interior of the vertebra.

I shall not swell the extent of the present papaer by pursuing farther the description
of the structure of the pelvis of the Dinornis, as exhibited in the present strikiing frag-
ment, but proceed to notice the other specimens of pelvic bones which have been enu-
merating as forming parts of the present collection.

The large portion of the right os innominatum, including the entire acetabulum
(Pl. XX. fig. 1.), must have belonged to a bird of rather smaller size than the one to
which the above-described portion of the sacrum belonged. The part of the ilium
before and above the acetabulum rises with a steep slope and a slight general concavity
to meet its fellow above the spinous crest of the anterior part of the sacrum : behind
the acetabulum the outer surface of the ilium is divided into two facets, the upper
one nearly horizontal, the lower one vertical, save where it arches out to the flat arti-
cular surface behind the acetabulum. The ridge dividing these two facets commences
anteriorly above the middle of ehe acetabulum, and describes a regular curve in its
course backwards, the convexity being downwards : in the Ostrich the correspond-
ing ridge forms two curves, meeting at an angle above the prominent articular sur-
face behind the aceabulum, and the convexity of both curves is upwards ; from the
angle and obsolete ridge extends down to the prominent articulation, and divides the ante-
rior from the posterior vertically concave surfaces of the ilium : in the great Dinornis
the corresponding surfaces are uninterruptedly continuous above the acetabular promi-
nence. The posterior wall of the acetabulum (f) is incomplete, as in other birds ; the
smooth articular surface is continued upon an oblong prominence above and behind the
cavity. The pubis (d),a slender bone, as usual in Birds, springs from a protuberance at
the lower part of the acetabulum. the ischium (e) is continued more directly from the
lower and back part of the cavity : a very slight ridge indicates the posterior boundary
of the notch for the tendon of the obturator internus, and the upper border of the notch

1 Cyclop. of Anat., art. AVES. p. 271. The Ostrich is the only exception to this rule with which I am
acquainted.