58 PROFESSOR OWEN ON THE 
The first sacral shows the transversely extended and concave articular surface of the 
centrum (Pl. XIX. fig. 1, ¢); the subcircular pit (ib. p) for the head of the rib is behind 
the middle of the side of the centrum, at its upper part; the inferior surfiice is ridged 
lengthwise; and a transverse low but sharp ridge defines the posterior boundary, the 
depressions in front of which indicate the hindmost origins of the subyertebral muscle 
(longus colli?). The anterior outlet of the neural canal (ib. 2) is subcircular in one spe- 
cimen, vertically elliptic in others, and 3 lines or less in transverse diameter. From the 
sides of the neurapophyses stretch out the strong buttresses of bone which blend with 
the under part of the ilia, giving off from the fore part of their base the preezygapophyses 
(ib. z), and from the back part of their apex the surface (ib. d), or part of it, for the 
tubercle of the last moveable rib, the ilium in the latter variety affording the rest of 
that surface. ‘The fore part of the strong neural spine (ib. ms) is roughened by a syn- 
desmotic surface ; it rises to a height of 14 lines, curving forward, and is confluent at its 
summit with the approximated anterior margins of the ilia. A continuous track of bone, 
forming a smoothly obtuse longitudinal ridge, represents the summits of the succeeding 
sacral spines (ib. fig. 2, 2s) to the hindmost vertebra of the series, without any trace of 
their primitive division; but this track rises, posteriorly, above the shallow channel on 
each side, in which are the foramina (ib. 0), indicating most of the constituent vertebre. 
The second sacral vertebra abuts against the ilium by a pleurapophysis (ib. fig. 1, 
pl2), as well as a diapophysis (ib. d2); but the former is a slender, straight filament, or 
narrow plate of bone, confluent at both ends. 
In the next two vertebre the pleurapophysis (ib. p/3&4) assumes more breadth and 
robustness, but is short and straight, abutting against the inner surface of the ilium an 
inch in advance of the acetabulum. ‘The first of these rib-buttresses inclines forward, 
and is completely confluent with the ilium; the thicker one (ib. p/ 4) has retaimed part 
of its primitive ligamentous attachment to the ilium: the proportions of both are sub- 
ject to some variety. 
These are succeeded by three or four vertebra in which the pleurapophysis is not de- 
veloped, the attachment to the ilia being by diapophyses only (ib. d d), which are short 
slender lamellee, directed upward and backward ; below and between them are the double 
orifices for the separate motory and sensory roots of the sacro-spinal nerves. In the 
next vertebra the pleurapophysis (ib. p/ 8) reappears, longer but more slender than in 
the fourth sacral, extending obliquely backward, and expanding at its extremity to abut 
against a prominence on the underside of the ilium, opposite the hind part of the 
acetabulum, with which prominence the rib has completely coalesced by an expanded 
end. The under part of all these vertebra is traversed by a sharp median longitudinal 
ridge, which is more feebly and interruptedly continued to near the end of the sacral 
series. 
Eight vertebra, abutting by diapophyses only (ib. d d) against the ilia, succeed the 
one last described; their coalesced bodies are less than half the breadth of those of the 
