in the Museum of Buenos Ayres. 91 
pared with that of the teeth of the Carpincho: it is peculiar to 
the Glyptodon, because no other animal equally large possesses 
teeth of this form. The zygomatic arch of this animal, besides 
being thick, was provided with a perpendicular prolongation 
which descended from underneath the eye, giving us a proof 
that it ground hard substances. This prolongation is only dis- 
covered in antediluvian animals, such as the Megatherium, Mylo- 
don, or the Scelidotherium *. 
As we only intend to describe the principal parts, I shall here 
conclude the cranium, and shall give.a description of the differ- 
ences in the teeth of the various species of the animal, they being 
the only parts which can be compared one with another. 
I have in my possession portions of three lower mandibles, 
two of which belong to G. clavipes, and the other (which is com- 
plete) to G. spinicaudus. The general form and the relations of 
the teeth are the same; but the form of the prisms in each tooth 
is a very little different. The sides of each rhombic prism of 
G. clavipes are a little curved into the interior of the prism ; but 
those of G. spinicaudus have a slight external elevation ; and for 
this reason the dental prisms in the former species appear to be 
thinner and more acute at the corners, and those of the second 
thicker and more obtuse. In the work of Dr. Lund (second 
part, tab. 35. f. 2,3 & 4) there are figures of two teeth which 
appear to exhibit a slight difference, as regards the form of the 
prisms, from those of my two species, demonstrating that there 
was a slight difference between the Brazilian species and those 
of Buenos Ayres. These teeth are from the upper jaw,—fig. 2 
being the first, and fig. 3 the last tooth on the left side. 
The neck of the Glyptodon comprises seven vertebree, as*in 
other Mammalia ; but only the first and the last are moveable, 
the other five being united into one solid mass; hence it results 
that the neck is very short and powerful. The first, or atlas, is of 
considerable size, and of the form occurring in other Mammalia; 
its two wings are laterally compressed, ascend posteriorly, and 
have three smooth and even slightly concave excavations for their 
articulation with the second vertebra. This (the axis) is short 
and united with the four following ones in one piece, which de- 
velopes in front a small tuberosity for its articulation with the 
atlas. At each side of this bone there is a strong prolongation 
inclined backwards, and before it four foramina for the trans- 
mission of nerves, which indicate the five united vertebre. 
There is another prolongation on the top of the arch above the 
vertebral column, which is also inclined backwards, and is ter- 
minated by three points. This particular bone is already known 
* [It is also visible in the existing Sloth.—TRANsL. | 
