376 
PROFESSOR OWEN ON THE MEGATHERIUM. 
base of the neural arch, which has descended lower upon the sides of the centrum ; 
and now we find another element — the pleurapophysis, pi — restored to the segment, 
but reduced to rudimental proportions and anchylosed at two points. Its vertebral 
end is bifid ; one portion, answering to the head of the rib, has coalesced with the 
side of the centrum (at p, fig. 5) ; the other, answering to the tubercle, has united 
with the under part of the diapophysis, d\ what may be termed the body of the rib 
is a short but broad rhomboidal plate (fig. 6, pi), projecting outward, downward and 
a little backvrard. The space intercepted between the pleurapophysis and diapophy- 
sis forms the canal, v, for the vertebral artery. 
The fifth cervical (Plate XVII. C &) differs from the sixth by its smaller dimen- 
sions, especially by its shorter spine, and by the diminution in the breadth of the 
pleurapophysis, which terminates by a thick obtuse end : it sends out, however, a thin 
plate forwards from its vertebral end. The metapophysis is a large obtuse tubercle, 
Plate XX. fig. 5, m s. 
In the fourth and third cervicals (Plate XXI. figs. 3 & 4) the neural spine is still 
more reduced, and, contracting from its base, assumes a triangular shape, fig. 4, ns. 
The anterior zygapophyses, fig. 3, «, are concave transversely, and look upward and 
inward ; the posterior ones, fig. 4, z, are convex, with the reverse aspect : the met- 
apophysis, m, continues to be developed as a distinct tuberosity, external and posterior 
to the prozygapophyses ; and the pleurapophysis continues to send forward the 
pointed plate from its fore-part, fig. 4, pi, its outer end, pi, being thick and tuberous, 
like the diapophysis, d, above. 
The dentata (Plate XXL figs. 1 & 2) has its spine extended in the antero-posterior 
direction, and of great strength, though low ; with a thick angular ridge projecting 
from its fore-part and overhanging the neural canal ; it is broad, flattened and almost 
vertical behind, and has a subbifid summit (Plate XX. fig. 5, 2 , and Plate XXL 
fig. 1, ns). There are no metapophyses and no anterior zygapophyses, but the ana- 
logous articular surfaces (figs. 41 & 42, xw) have descended upon the antero-lateral 
parts of the coalesced centrum of the atlas or ‘ odontoid process,’ and are adapted to 
corresponding surfaces of the bases of the neural arch of the atlas. The posterior 
zygapophyses, z', are wide apart, and are convex. The diapophysis, d, is short and 
obtuse ; the pleurapophysis, pi, still developes its anterior angle, Plate XXL fig. 2, pV. 
The fore-part of the odontoid process, o, is a rounded tuberosity, on the under sur- 
face of which is the oval, slightly convex surface for articulating with the hypapo- 
physis (Plate XX. fig. 3, 0 , Jiy), which has coalesced with the neural arch, ns, of the 
atlas, and is commonly called the ‘body of the atlas.’ The under surface of the 
centrum of the dentata developes a hypapophysial ridge. 
The atlas, viewed from behind, as in Plate XX. fig. 3, is a large, transversely ob- 
long, subdepressed, shuttle-shaped bone, perforated by a large aperture, quadrate 
below for its detached centrum the ‘odontoid process,’ arched above for the spinal 
cord. 
