372

342

so defined, but is undulated by a rising, convex lengthwise, parallel with the mid third
of the costal borders.

The figures of the sternum of Aptornis defossor in Pl. LXXXIX., being of the natural
size, preclude the necessity of further admeasurements.

There is no pneumatic perforation in any part of the sternum, which agrees, in this
respect, with that provisionally referred to the smaller species of the genus.

The generic distinction between Aptornis and the extinct gigantic Ralline bird called
Cnemiornis is strongly expressed by the sternum. In Cnemiornis the ridged repre-
sentative of the keel 1 subsides much nearer the fore part of the bone ; the costal border
is relatively longer, and includes articular surfaces for seven sternal ribs ; the body of
the bone appears to retain its breadth for a greater extent as it recedes than in Aptornis
defossor. The portions of sternum picked up with the other bones of Cnemiornis
enable the above comparisons to be instituted, but they are not sufficient to give a
satisfactory idea of the shape of the entire sternum in that genus.

The pelvis of Aptornis defossor, figured of the natural size in Pl. LXXXIX. fig. 1
and Pl. LXXXVIII., exhibits a slight deformity or departure from the normal symme-
trical figure at its fore part, which is somewhat bent to the right - the twist, which
chiefly affects the neural spine and ilia, being most conspicuous when the bone is viewed
from the front and from below, as in fig. 2. Pl. LXXXVIII. It is doubtless an indi-
vidual variety, and indicates the bird from which it was derived to have been aged.

The articular surface of the centrum of the first sacral vertebra (PL. LXXXVIII.
fig. 2, s 1) is transversely extended, measuring in that diameter 1 inch 4 1/2 lines, the
vertical diameter at the middle being 5 lines ; but this extends to 7 lines on each side
beyond the neural canal, which is circular, and only 3 1/2 lines in diameter at its anterior
outlet. The prezygapophyses (Pl. LXXXIX. fig. 1, z) are large, with their outer borders
bent strongly forward ; the right is the largest, partaking of the asymmetry above noticed.
The articular surface, concave transversely , is vertically plane, or rather convex. The
fore part of the neural spine shows a rough surface for ligamentous attachment along the
lower half of its extent ; this surface expands to a breadth of 4 lines, then contracts to a
point, beyond which the spine presents a sharp anterior margin to near the summit,
where it again expands and coalesces with the contiguous parts of the iliac bones. A
deep and narrow ilio-neural fissure extends backward on each side the spinal plate.

The neuropophysis, developing the zygapophyses, is coextensive with the centrum,
and expands beyond that element into a broad depressed diapophysis, confluent above
with the ilium (Pl. LXXXVIII. fig. 2, i): this process forms at its hinder angle a
roundish flat surface (Pl. LXXXIX. fig. 1, d) for the tubercle of the first free sacral
rib. The articular surface for the head of that rib is large and flat on the left side
(ib. fig. 1, pl), but is small on the right side, partaking of the asymmetry above noticed.
The under surface of the centrum is carinate ; the keel beginning 2 lines behind the

1 Pl. LXVI. fig. 9, s.