18 



for the ninth pair of haemapophyses which terminate each by a single convex condyle. 

 It is possible that a more simplified sterneber may have intervened between the 

 haemapophyses of the tenth segment. 



In the three segments of the trunk, Plate I. L i, a, ^ 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 haemapophyses are repeated. 



In the third segment the pleurapophysis, pi, fig. 3, Plate IX., and haemapophysis, 

 ib. h, are anchylosed together: both are shortened, but the pleurapophysis in a 

 greater degree ; this retains its three articular surfaces, c", n", d", on the head, neck 

 and tubercle ; and the haemapophysis, h, has its double condyle, s 1 , s", 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 I. D a) the neural spine is increased in 

 height, and the metapophysial tubercle is diminished in size. The vertebral, pi, and 

 sternal, h, parts of the rib (Plate IX. 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 {pi, fig. 2) is diminished in length, but 

 increases in breadth to its place of coalescence with the haemapophysis (h) ; the 

 convex articular surface (fig. 2b, V) on the outer condyle of the haemapophysis is not 

 divided : the inner condyle, d, is much reduced and has only a small articular 

 surface. 



The first dorsal segment (Plate I. D i) 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 IV. fig. 5. D i, ns. The anterior mar- 

 gin 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 IX. pi, fig. 1) is much reduced in length, and is con- 



