PROFESSOR OWEN ON THE MEGATHERIUM. 
377 
The thinnest and smallest part of the ring of the atlas is formed by the hypapo- 
physis, hy, which has coalesced with part of the bases of the neural arch, ww, and 
has supplanted, as it were, the proper centrum, o, Plate XXL figs. 1 & 2, which has 
remained anchylosed to that of the axis. The upper surface of the hypapophysis 
presents a shallow articular surface, o, Plate XX. fig. 3, for that centrum to rest and 
turn upon. The hinder half of the base of the neurapophysis developes, on each side, 
a slightly concave, subcircular, articular surface, z', for a moveable articulation with 
that on the side of the odontoid, zn, fig. 1, Plate XXL The atlas is perforated 
anterior and external to this by a foramen, Plate XX. fig. 1, s, answering to that called 
‘foramen alare posterius’ in the Horse, in which it gives passage to the posterior 
branch of the occipital artery ; in the Megatherium the foramen or canal is bridged 
over by a narrow oblique bar of bone, dividing its external outlet into two, r & s, 
and through the hinder, s, of the divisions it is probable that a branch of the second 
spinal nerve may have passed. 
The diapophysis, d, is a broad, depressed, rounded aliform process, with a protu- 
berance from its under and back part, like the rudiment of a pleurapophysis, pi. 
Anterior to this process the under surface of the diapophysis is deeply and widely 
excavated and perforated by the vertebral artery, the canal for which, opening upon 
the upper surface of the diapophysis, is then continued obliquely inward, perforating 
at q, fig. 4, Plate XX. the upper part of the neural arch, just within the upper part of 
the condyloid concavities. A large part of the canal for the first spinal nerve, fig. 1, v, 
opens into the outer commencement of the vertebral canal, and answers to that 
called ‘ foramen alare anterius ’ in the Horse, which transmits the inferior branch 
of the first spinal nerve as well as the anterior branch of the occipital artery. The 
condyloid concavities, fig. 2, nz, are semioval, large and deep, and occupy nearly the 
whole of the anterior surface of the neural arch, being separated above by a rough 
tract of three inches’ extent, upon which the vertebral canals open. There is a tri- 
angular rough surface at the back and inner part of each condyloid concavity. 
Such are the modifications of the different cervical vertebrse of the Megatherium. 
With regard to the dorsal vertebrse, their chief characteristics may be briefly recapi- 
tulated as follows : — 
The first dorsal vertebra is distinguished by the confluence of the neuro-costal and 
dia-costal surfaces, and by the superior height of the spine. 
The second to the fifth dorsals inclusive, like the first, have only the ordinary pair 
of zygapophyses before and behind, but have the neural and diapophysial surfaces 
for the rib distinct. 
The sixth dorsal is recognizable by having a third median zygapophysis behind, 
but not in front. The seventh to the twelfth dorsals inclusive have the three zyg- 
apophyses both before and behind. The thirteenth dorsal has the median zygapo- 
physis in front but not behind : the costal surface has disappeared from the diapo- 
physis. The fourteenth dorsal has only the ordinary pair of zygapophyses before and 
3 B 2 
