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PROFESSOR OWEN ON THE MEGATHERIUM. 
In the three segments of the trunk, Plate XVIL L i, 2 , 3 , succeeding the last of the 
dorsal series, both pleurapophyses and haemapophyses are wanting as distinct ossified 
parts, and those segments are reduced to the coalesced elements, constituting the 
‘lumbar vertebrae’ of Human Anatomy. The accessory articulations between the 
parapophyses and anapophyses are continued in these vertebrae, which do not become 
anchylosed together in the Megatherium as in the Mylodon. 
I next proceed to trace the modifications of the segments as they recede from the 
typical one in the opposite direction or towards the head. 
The fourth dorsal segment much resembles the fifth, which has been taken as the 
type; the pleurapophyses are shorter, especially at their cervix; but the complex 
articulations of these and of the hmmapophyses are repeated. 
In the third segment the pleurapophysis, yl, fig. 3, Plate XXV., and haemapophysis, 
ib. h, are anchylosed together; both are shortened, but the pleurapophysis in a 
greater degree; this retains its three articular surfaces, c"., d", on the head, neck 
and tubercle ; and the haemapophysis, h, has its double condyle, d , x", at the sternal 
end, fig. 3 b, the inner one being single, the outer one divided by a narrow groove 
into an anterior and a posterior convexity. The concave border of the rib is less 
produced than in the fifth segment. 
In the second dorsal segment (Plate XVII. D 2 ) the neural spine is increased in 
height, and the metapophysial tubercle is diminished in size. The vertebral, yl, and 
sternal, h, parts of the rib (Plate XXV. fig. 2) are anchylosed, and both are shortened : 
the former retains its three articular surfaces on the head, c", neck, n", and tubercle, 
d", that on the tubercle being the largest, and being partially divided into two con- 
vexities (fig, 2a, d). The pleurapophysis {yl, fig. 2) is diminished in length, but 
increases in breadth to its place of coalescence with the haemapophysis (A) ; the 
convex articular surface (fig. 2b, s") on the outer condyle of the haemapophysis is not 
divided ; the inner condyle, s', is much reduced and has only a small articular 
surface. 
The first dorsal segment (Plate XVII. D 1 ) is remarkable for the superior height 
and antero-posterior extent of the neural spine, the summit of which expands into 
a broad flat subtriangular rough surface, Plate XX. fig. 5. D 1 , ns. The anterior 
margin of the spine is sharp and produced. The anterior zygapophyses are not so 
near each other as in the succeeding vertebrae; and they are continued outwardly 
upon the base of the metapophysis, which is here more distinct from the diapophysis 
than in the succeeding vertebrae; and the articular surfaces of the zygapophyses are 
slightly concave transversely. The costal concavity below the diapophysis is con- 
tinuous with the smaller articular surface upon the side of the neurapophysis. 
The pleurapophysis (Plate XXV. />/, fig. 1) is much reduced in length, and is con- 
fluent below with a short, thick, broad, subquadrate haemapophysis, h. The short neck 
of the rib terminates in a small obtuse end without any distinct articular surface : this 
end seems to have been imbedded in the ligamentous mass between the seventh cer- 
