SOME EXTINCT ARGENTINE MAMMALS 89 
of plates, in the glyptodons the whole structure is welded 
into a single piece. It must not, however, be supposed that 
this carapace consists of a single solid dome of bone, as, 
if it did, there would, of course, be no possibility of growth. 
On the contrary, the carapace is composed of polygonal 
or rhomboidal plates articulating at their edges, and thus 
allowing of free growth. In very old individuals a consider- 
able number of these plates may, however, become com- 
pletely fused together. During life these bony plates were 
covered with small horny shields, as in the living arma- 
dillos, and they frequently show incised lines formed by the 
lines of union between such shields. For instance, in the 
members of the typical genus of the group, or ring-tailed 
glyptodons, each bony plate was smooth and polygonal in 
shape, while the lines indicating the borders of the horny 
shields take the form of a rosette. Another important 
point of difference from the armadillos is to be found in 
the contour of the skull, which is short, deep, and rounded, 
instead of being long, flattened, and pointed at the muzzle. 
Then, again, whereas the armadillos have small cylindrical 
teeth, those of the glyptodons are large and fluted at the 
sides, with their grinding surfaces marked by the aforesaid 
sculpture; while the whole series is in close contact, 
and forms one of the most efficient grinding machines 
imaginable. 
To support the enormous weight of the carapace, which 
in some of the larger kinds is considerably more than an 
inch in thickness, special modifications are needed in the 
internal skeleton. Here we find that nearly the whole of 
the vertebrae are welded together, so that a large portion 
of the backbone forms a continuous solid tube. The ver- 
tebrae of the neck are also very short, and may be partially 
united, so that the movements of the head must have been 
