330 A NEW MYLODON. 
concave, with the general plane of their surfaces parallel with the frontal plane. 
They are borne directly over the pedicels, those of the 3d and 4th with their 
greatest diameter longitudinal, and of the 5th to 8th with this diameter trans- 
verse to the direction of the spinal column. This feature, together with the 
imbrication of the posterior over the anterior ends of the neural spines limits 
greatly the sidewise movement of the neck vertebrae, but allows much freedom 
of motion in a vertical direction. 
With the ninth vertebra the cranial facets become shifted medially so as 
to occupy nearly the entire length of each side of the arch. Their plane thus 
makes an angle of nearly 45 degrees with the transverse axis. The facets them- 
selves measure some 50 mm. in long diameter by 20 in anteroposterior length. 
On the tenth and succeeding vertebrae the cranial facets tend to increase 
in their anteroposterior diameter and to diminish in transverse extent so as to 
become more or less irregularly rounded from the 13th to 20th. This is accom- 
panied by a flattening of the anterior portion of the vertebra, so that from the 
13th onward, these facets look directly upward. The 22d and 23d vertebrae 
show enlarged facets, those of the former with a slightly different angle of slope 
at their ventral half, those of the latter practically divided into two contiguous 
oval facets, the lower partly in advance of the upper and facing slightly outward. 
The 24th vertebrae is lost, but judging from the caudal articular facets of the 
23rd, there were two wholly separate cranial articulations on each side. 
A remarkable structure occurs on the 17th vertebra in the shape of a third 
caudal articular facet with the form of an elongated oval, situated on the pos- 
terior face of the dorsal spine 30 mm. from its tip and 11 mm. from the paired 
caudal facets. This articulates with a third median facet of similar shape on 
the front of the 18th vertebra at the base of the neural spine. The facet on the 
17th vertebra measures 39 mm. in length by 14 mm. in breadth. The third 
posterior facet of the 18th vertebra is broader and not so long (86 X 17 mm.) 
and 39 mm. from the tip of the spine. That of the 19th vertebra is again longer 
(52 x 18 mm.) and differs further in that its lower end extends between the tips 
of the two lateral caudal facets. In the 20th and 21st vertebrae the condition 
is similar, except that the facet becomes successively smaller (30 X 18 mm., and 
25 < 14mm. respectively). The 22d vertebra (Plate 4, fig. 17) has the anterior 
third facet, corresponding in size to the posterior facet of the preceding vertebra, 
but it has none on its posterior surface. This additional articulation is thus 
present on the posterior face of the 17th and the anterior face of the 22d verte- 
brae, and on both faces of the four intervening. According to Owen (1842) this 
