378 
PROFESSOR OWEN ON THE MEGATHERIUM. 
behind, but may be distinguished from the second, third, fourth and fifth dorsals by 
the absence of the costal articulation on the diapophysis, and of that on the upper 
and hinder angle of the centrum. The fifteenth dorsal has an anapophysis on each 
side with an articular surface, and has only the costal articulation on the neurapo- 
physis. The sixteenth dorsal has on each side, at the fore part of the neural arch, a 
parapophysis with a superior articular surface, and behind, an anapophysis with an 
inferior articular surface. But it differs from the lumbar vertebrae by the costal sur- 
face on the neurapophysis. 
Having now described the principal characters of those segments of the skeleton, 
the centrums and neural arches of which are comprehended in Anthropotomy under 
the term of ‘ true vertebrae,’ on account of their freedom of motion on each other, I 
next proceed to the description of the ‘ false vertebrae and first, of those that, being 
anchylosed together, form the ‘ sacrum.’ 
This part of the skeleton includes five vertebrae (Plate XXIIl. 1-5), which are not 
only anchylosed to each other, but to both the iliac and ischial bones : the length of 
the sacrum is 22 inches, its extreme breadth across the fifth vertebra, fig. 1, ds, is 
20 inches. The centrum of the first vertebra (Plate XXII. c) presents a transversely 
oblong, subquadrate, flattened, articular surface for that of the last lumbar vertebra, 
with its mai'gin a little produced forwards, and developed below into a pair of rough 
ridges, * *. The neural arch overhangs this surface, and developes a metapophysis 
from the fore part of each side of its base, with a broad articular surface on its under 
part, and a similar surface above (Plate XXIIl. fig. 1, z), representing the anterior 
zygapophysis ; the two surfaces meeting at a right angle at their inner borders. 
The broad diapophysis of the first sacral, 1 , is perforated by a small subvertical 
canal, d', at its confluence with the ilium, and is separated from the corresponding 
part of the next diapophysis by a larger orifice, o 1, which is the first of the four 
superior or posterior sacral outlets. I’he neural arch of the first sacral vertebra, n 1 , 
is separated from that of the second, n 2, by a narrow transversely elongated ellip- 
tieal vacuity. The neural arches of the three succeeding vertebrae are completely 
confluent: a pair of triangular closely approximated apertures, w 4, divides the base 
of the neural spine of the fourth, ns 4 , from that, ns 5, of the fifth sacral vertebra. The 
neural spines of the first four sacral vertebrae have coalesced into a strong vertical 
ridge, ns^ ns i, increasing in thickness as it extends backwards, and being there from 
two-thirds of an inch to one inch and a half thick across the broken summit. The 
second posterior sacral canal, o 2, intervenes between the diapophysis of the second 
and that of the third vertebra. The metapophysis, m 4, of the fourth appears as a low 
angular tubercle above and a little behind the diapophysis of the third sacral, 3. The 
diapophysis of the fourth sacral, d i, extends outwards beyond the ilium, as a sub- 
depressed broad process with a rough free extremity; the back part of the process 
coalesces with a similar but stronger and longer diapophysis of the fifth sacral, di, 
from tlie fore-part of the base of which a tuberous metapophysis, m 5, projects upward 
