PROFESSOR OWEN ON THE MEGATHERIUM. 
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plenrapophyses, j)l, j)l, being elongated and bent down, and the hsernapophyses, hJi, 
removed from the centrum and articulated to the ends of the plenrapophyses, and, 
by a double synovial joint, .yV, to the haemal spine or sternal bone h.s. 
The coalesced centrum and neural arch constitute the so-called ‘dorsal vertebra,’ 
and the one selected is the fifth of that series counting backwards. 
The centrum, c, or body of the vertebra, is wedge-shaped, with its base upwards, 
forming the floor of the capacious neural canal, and the sides — slightly concave 
lengthwise, almost flattened vertically — converge to the inferior surface, which is 
formed by an obtuse ridge: the centrum expands slightly at its articular ends, and 
so that the contour of the anterior one is rather oval than trihedral ; this surface is 
slightly depressed at its middle, slightly convex in the rest of its extent ; the posterior 
articular surface is larger than the anterior one and flatter, but is also a little 
depressed at the middle: the two upper angles of the hinder end, probably con- 
tributed by the neurapophyses in the development of the vertebra, are slightly pro- 
duced and truncate, offering each a smooth, flat, small subcircular surface, d, for 
the head of the rib of the preceding vertebra; the corresponding part of the rib of 
the present segment is marked c". The neurapophyses, w, n, rise each by a slender 
base which has coalesced with the anterior half of the upper and outer angle of the 
centrum : they diverge from each other and expand as they rise ; then, developing 
some articular surfaces and exogenous processes from their outer surfiice, arch 
towards each other, increasing rapidly in antero-posterior extent, and coalesce above 
the neural canal ; where they support the zygapophyses, z, z, and the thick and strong 
neural spine ns. The roof of the neural arch, thus formed, projects some way beyond 
the anterior surface of the centrum, and extends almost to the posterior surface. 
The inner surface of the neural arch is as remarkable for its even smoothness, as the 
outer surface is for its various prominences and depressions. The outer side of the 
basal half of the neurapophysis supports a large elliptical articular surface (w/), con- 
cave from above downwards and backwards : the overhanging fore-part of the arch 
supports the two flat oval anterior zygapophyses, z,z, the articular surfaces of which 
look almost directly upwards ; on the under surface of the back part of the arch are 
the two posterior zygapophyses looking almost downwards, and between these is a 
rough longitudinal prominent ridge. The neural spine is moderately long, subcom- 
pressed and subtrihedral, with a sharp anterior margin, smooth sides, and a rough 
thick posterior surface, developing a median longitudinal ridge: the summit expands 
into a rough triangular almost flattened surface. 
For the convenience of describing and comparing the different exogenous processes 
developed from the neural arch in the class Mammalia, I have indicated them by 
single-worded names; having found that, although they varied much in size and a 
little in relative position, when traced through the series, they could be identified 
from species to species. 
The most common and constant of these processes is that which usually stands 
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