PROFESSOR OWEN ON THE MEGATHERIUM. 
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and forward. The third, o 3 , and fourth, 0 4, upper sacral outlets are wider apart than 
the second and first; the sacrum expanding posteriorly. The back and under part 
of the diapophysis of both the fourth and fifth vertebrm coalesce with the ischium 
and with the thick and strong parapophysis extended from the side of the centrums. 
The neural arch of the fifth sacral developes a pair of posterior zygapophyses, Plate 
XXIII. z',z', with a flat surface looking outward and a little downward, and with 
the lower angle continued upon a small rough subarticular surface. The posterior 
surface of the last sacral vertebra, Plate XXIII. fig. 2 , is on the same vertical parallel 
as the posterior zygapophyses ; it is nearly flat and transversely elliptic. The neural 
canal of the sacrum, the anterior aperture of which is 3 inches in vertical and 4 inches 
in transverse diameter, expands in the sacrum, and opens below by three wide fora- 
mina on each side : of these the first and second are of great size : into the second 
foramen the third upper sacral canal leads : the third lower sacral foramen, which 
is the smallest, corresponds with the fourth upper one: the fifth canal for the fifth 
pair of sacral nerves broadly grooves the back part of the parapophysis and side of 
the centrum. The posterior aperture of the neural canal is 2 inches in vertical and 
4 inches 3 lines in transverse diameter. Both diapophyses and parapophyses of the 
first three sacral vertebrae coalesce with the ilia. The sacrum is concave below both 
transversely and lengthwise. 
The tail of the Megatherium was of great strength : it is so long as to touch the 
ground when the trunk is raised at an angle of forty-five degrees from the horizontal 
position : it includes eighteen vertebrae, which progressively diminish in size from the 
first to the last, Plate XVII. Cd, i-is. 
The first vertebra, Plate XVIII. fig. 2 , is remarkable for the length and strength of 
its diapophyses, d, which are expanded at both ends, and, like those of the sacral ver- 
tebrae, are probably lengthened out by connate or coalesced pleurapophyses, The 
base of the process, dp, extends from the side of the centrum to the base of the neural 
arch, is widely excavated behind for the passage of the first pair of caudal nerves, 
and is subcompressed before it expands into its rugged free termination. These pro- 
cesses are shorter than those of the last sacral vertebra. The neural canal, n, is tri- 
angular, 3 inches in vertical and 3 inches 9 lines in transverse diameter. The neural 
arch developes two posterior zygapophyses, z', with their articular surfaces looking 
downwards and outwards : two anterior zygapophyses, z, with their articular sur- 
faces looking upwards and inwards, these being strengthened by a strong tuberous 
metapophysis, m, on their outer side: the spine, ns, is of moderate length, carinate 
behind, obtuse and slightly expanded above. From the under part of the trans- 
versely elliptical centrum are developed two hypapophyses, hy, each with an oblong 
articular surface, Plate XXVI. fig. 6 , hy, to which is joined a hsemapophysis, Plate 
XVIII. h. Each heemapophysis is a long slender conical bone, with an articular 
surface at each angle of the base, hy’, hy", and an obtuse slightly inflected apex : the 
inner side of the bone is slightly concave, the outer one convex transversely : a rough 
