380 
PROFESSOR OWEN ON THE MEGATHERIUM. 
tuberosity dividing it from the inflected apex. The essential differences between the 
first caudal segment and the dorsal one delineated on the same Plate are, that the 
pleurapophysis, pi, is short and anchylosed to the diapophysis, the heemopophyses, h, 
articulate with the centrum, and the haemal spine is absent, in fig. 2. 
The second caudal vertebra (Plate XXIV. figs. 1 and 2) differs from the first in 
having an anterior, fig. 2, hy, as well as a posterior, ib. hy', pair of hypapophyses ; 
and in the confluence of the hsemapophyses, fig. 1, A, at their apices forming the so-called 
“^chevron bone’ {os en chevron, Cuvier). This vertebra is smaller than the first caudal 
in all its parts except the heemapophyses, and in all its dimensions except the vertical 
diameter, which is due to the development of the coalesced parts of those elements 
into a long and strong hsemal spine, hs. The anterior hypapophyses, fig. 2,}iy, which 
are the smallest, articulate with the surface on the back part of the base of the haem- 
apophyses of the first caudal vertebra : the posterior hypapophyses, hy’, which are 
more oblong and closer together, articulate with the anterior 'and larger pair of sur- 
faces, hy’, of their own haemapophyses, fig. 1, h. There is a strong rough tuberosity 
projecting backwards external to each of the anterior hypapophyses. The posterior 
articular surface is, in the present instance, developed only on the right haemapo- 
physis, fig. 1, hy”-, on the left it is represented by a rough tubercle. 
From the third to the fifth caudal vertebrae inclusive, the proximal end of each 
haemapophysis has both the large anterior transversely oblong surface for its own 
centrum, and the smaller subcircular posterior surface for the next centrum : the 
spine, or coalesced portions, of the third pair is the longest in the caudal series ; be- 
yond this it progressively diminishes. In the sixth caudal vertebra the haemapo- 
physes have a rough protuberance instead of the posterior articular surface. After 
the eighth the protuberance subsides to a rough ridge. In the eleventh caudal 
(Plate XXIV. figs. 3-6) the distal end of the coalesced and shortened haemapophyses, 
hy, is truncate, and as broad as the divided bases. The under surface of the corre- 
sponding centrums of the sixth to the eleventh caudal offers the articular surface on 
the anterior pair of hypapophyses, fig. 6, hy ’ ; the posterior pair, ib. hy, are rough 
tuberosities. The posterior zygapophyses (figs. 3, 4, 5, z’ z’) retain their articular 
surfaces to the tenth caudal: in the eleventh they are mere angular projections. The 
metapophyses, 7nm, are continued to the fourteenth caudal. The neural spine is 
reduced to a low tuberosity on the thirteenth caudal (fig. 7, ns) \ the neural arch con- 
tinues complete to the sixteenth, fig. 8, n: in the seventeenth, fig. 10, the neurapophyses, 
n, are mere exogenous ridges, bounding the sides of an open neural groove. The 
liaemapophyses are continued to the fourteenth vertebra: two pair of rough low 
hypapophysial tubercles, hy, hy, fig. 9, continue to be developed to the sixteenth : 
they subside on the penultimate caudal, fig. 11, in which the diapophyses are repre- 
sented by an obtuse ridge on each side of the centrum. In the last centrum, tigs. 12, 
13, all the processes have disappeared, and it presents the form of a low rounded 
cone, with a smooth concave pentangular base, fig. 13, and a rough tuberous 
