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 nintli 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 (36 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 X 14 mm. 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 a,nterior face of the 22d verte- 

 brae, and on both faces of the four intervening. According to Owen (1842) this 



