PROFESSOR OWEN ON THE MEGATHERIUM. 
371 
for the concavity on the neurapophysis of its own vertebra; and is indented behind 
where it joins the tubercle. The oval convex articular surface, d", for that on the 
diapophysis, is situated on the upper and towards the back part of the tubercle. The 
body of the rib is moderately convex on its outer side, more convex transversely on 
its inner side, where the convexity is bounded by a groove on each side, extending 
half-way down the rib, near its rather sharp margins : in its lower half the rib be- 
comes a little broader and less thick. The outer surface of the rib is well marked 
by grooves and ridges for muscular attachment ; the best-developed eminence being 
at a short distance from the tubercle ; and the largest and deepest groove being 
behind the tubercle. 
The hsemapophysis, or ‘sternal rib,’ h, is a straight subcompressed bone, with a 
very irregular surface, which is somewhat convex on the inner side, but is traversed 
by strong oblique ridges with intervening deep and wide channels on part of the outer 
side. The surface of junction with the pleui’apophysis is a very rough and irregular 
one for ligamentous union : the opposite end of the hsemapophysis divides into two 
convex condyles, s' s', separated by an oblique, deep and rather narrow groove; the 
outer condyle projects further than the other; on the shorter one the articular sur- 
face passes continuously from one side to the other, describing a semicircle ; on the 
longer condyle the articulation is divided by a median constriction into two oval con- 
vex surfaces. One half of each of the condyles articulates with a corresponding con- 
cavity on the ‘sternal bone’ {hs) of its own segment, the other half of each condyle 
with the contiguous sternal bone. ' 
The sterneber or sternal bone, completing, as ‘ haemal spine,’ hs, the typical seg- 
ment in question, is a cuboid piece, divided into an outer or peripheral, and an inner 
or central portion. The outer portion is subpentagonal, having its four corners exca- 
vated by as many concavities for the haemapophysis ; the two upper concavities, s" s" , 
being divided by a flat rough tract, the two lower concavities by a rough tuberosity. 
The outer surface is flat and rough. The inner portion, or that next the cavity of 
the chest, is larger than the other and has a hexagonal contour; the four angular 
concave articular surfaces, s", s", for the hsemapophyses being separated, at the sides 
of the bone, by rough tracts ; and, above and below, by a fiat articular surface, hs, by 
which the bone articulates with contiguous sternal bones. The inner surface of this 
portion is flat and rough, having been apparently covered by a strong aponeurosis in 
the living animal. Thus the whole bone presents not fewer than ten articular sur- 
faces, viz. a flat semicircular one above or in front of, and a similar one behind, the 
posterior division ; and two concave articulations, s",s", on each side of both divisions 
for the double condyles of two pairs of heemapophyses. 
The chief modification in the sixth segment of the chest is the development of a 
third articular surface at the back part of the base of the spine, between the two 
posterior zygapophyses ; and the somewhat greater production of the ridge which 
stands out from the fore-part of the base of the spine between the anterior zygapo- 
