ON A NEW SPECIES OF MACRAUCHENIA (M. BOLIVIENSIS) 407 
confirms the conclusions arrived at by the examination of the more 
perfect fossils. It corresponds very well with the posterior half of 
the centrum of the penultimate lumbar vertebra of AZ. Patachonica 
in form, and with the corresponding vertebra of Auchenia in size ; 
but the crest into which the middle of its under surface is raised, 
and which is still sharper than that in the Patagonian species, 
diagnosticates it at once from any of the lumbar vertebrae of the 
Llamas. 
The transverse diameter of the articular face is 1°1 inch, its vertical 
diameter o'9. The corresponding measurements of the antepenulti- 
mate lumbar vertebra of J/. Patachonica are 3:0 inches and 2°1; so 
that, as in other bones, the proportions of diverse diameters of the 
same bone are not the same in the two species. But as the trans- 
verse diameters of the cervical vertebra of the two species are nearly 
as 1:3, and the transverse diameters of the lumbar vertebre are, 
also, nearly in the ratio of 1: 3, it would seem as if the different 
regions of the vertebral column of the two species exhibited the same 
proportional correspondence to one another. 
The skull—As no part of the skull of Macrauchenia Patachonica 
has yet been discovered (with perhaps the exception of part of the 
lower jaw), a great interest attaches to every fragment which promises 
to throw light upon this part of its organization; and I therefore 
make no apology for dwelling at some length upon the characters of 
the two very imperfect and mutilated portions of the cranium which 
turned up among the specimens submitted to me by Mr. Forbes. 
The one of these (Plate VI. [Plate 32.] fig. 3) consists of rather 
more than half of the occipital segment of the skull, and exhibits the 
whole of the supra-occipital bone, with its strong occipital crest, a 
part of the parietal with the sagittal crest, the greater part of the 
right paramastoid process, and the entire right occipital condyle. 
As I have already remarked, the sutures are obliterated: and this 
is true, not only of those which ordinarily exist between the elements 
of the occipital bone in young mammals, but of the lambdoidal 
suture, which usually persists for a longer period. The occipital 
foramen must, when entire, have had a depressed-oval form, the short, 
vertical axis of the oval being about 06 of an inch long. The face 
of the bone above it inclines upwards and forwards, at an angle of 
about 50° with the base of the skull, and presents a sharp ridge in the 
middle line, on either side of which the surface of the supra-occipital 
element slopes with a slight convexity outwards and forwards, at the 
sides and below ; while, above, it becomes concave by passing almost 
vertically upwards in the middle line, and laterally, bending upwards 
