PROFESSOR OWEN ON THE GENUS DINORNIS. 237 
given of the atlas of Dinornis robustus, of the natural size. In this species the greatest 
length of the vertebra, at the base of the neural arch, is 15 millim., the greatest breadth 
at the same part is 30 millim., the height of the vertebra is the same; the diameter of 
the cup for the occipital condyle is 15 millim. In Dinornis parvus the corresponding 
admeasurements are 10 millim., 20 millim., and 10 millim. respectively (Pl. LI. figs. 2, 
3, 4). 
The proper centrum (ib. figs. 5, 6,7, ¢') has coalesced with that of the second 
vertebra (ib. ¢”), its anterior convexity forming the ‘ odontoid process,’ and projecting 
into the canal formed by the confluent hypapophysis and neural arch of the atlas. The 
hypapophysis (ib. fig. 2, hy) is, as usual, wedge-shaped, with the base downward, 
as in Ichthyosaurus; the apex is truncate, and deeply emarginate, to receive the odontoid 
process. At the back part of the transversely extended and fiattened base (ib. fig. 4) is 
a feeble rudiment of the more developed process in Dinornis robustus (loc. cit. pl. liii. 
fig. 6, hy). The postero-lateral extensions give confluent attachments to the slender 
columnar processes of the neurapophyses (ib. fig. 2, ) which bound outwardly the 
large vertebrarterial foramina (ib. figs. 3, 4, v). The fore facet of the hypapophysis 
is deeply cupped (ib. fig. 3) for articulation with the occipital condyle. The hind 
facet (fig. 4, hy) gives a smooth, almost flat transverse surface to the corresponding 
anterior facet of that of the axis (ib. fig. 8, 7). . 
The base of each neurapophysis of the atlas has coalesced with the hypapophysis, 
and there sends mesiad a short pointed process, tending to circumscribe the atlantal 
cup below and the wide neural cavity above. Each neurapophysis then curves 
upward, outward, and inward, coalescing with its fellow above the neural canal (n’), 
over which it arches. From the hind part of the base of each neurapophysis extends 
a short strong process, developing a tuberosity externally (z') and a flat, articular, 
subcircular surface mesially, which joins the prezygapophysis of the axis. The roof 
of the neural arch is a broad bridge-like plate of bone, without trace of neural spine. 
In this respect it resembles that part in Dinornis maximus, but differs in shape and 
extent. The mid line of the bridge in J). maximus equals one half of the transverse 
extent; in D. parvus it equals one third. . The fore border of the bridge is straight in 
D. parvus, but is convex in D. maximus. The hind angles supporting the postzyga- 
pophysial surfaces (z) are relatively more produced in D. maximus, the upper surface 
of the atlas presenting a horse-shoe figure. 
In Dinornis robustus the marked difference from both the above species is seen in 
the non-union of the sides of the neural arch, leaving a longitudinal fissure at the 
upper mid line (Joc. cit. pl. liii. figs. 4, 5, c); it is possible that the specimen yielding 
those figures may have come from an immature individual. The more complete ossifi- 
cation of the vertebra in D. parvus is one of many similar evidences of the maturity of 
the bird yielding the subject of the present description. 
