204 THE DESEADO FORMATION OF PATAGONIA 
the head shield, which consists of nineteen or twenty-one 
definitely arranged head plates, the anterior ones being 
developed into horn-like projections. The plates of the 
carapace are wide, thin, and unique in each having two 
to four wide shallow pits on the exposed surface. We found 
the genus rare, only two isolated plates turning up. From 
the Deseado material Ameghino has made three species: 
P. protervus, of very large size; P. undulatus, of moderate 
size, with the median figure accentuated and ending in two 
pits and with piliferous depressions on the margin; and P. 
depressus, of the same size as the foregoing, with a faint 
central figure, often four pits on the exposed surface and 
no piliferous pits on the margin. We found but one species, 
one plate of which combines characters of both the last 
two as described, so that I feel that there should be but 
two species, P. protervus and P. undulatus. 
Peltephilus undulatus Ameghino 
P. undulatus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 509. 
P. depressus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 510. 
One of the plates we found has the rough surface, obscure 
figure, two pits on the median part of the surface, and 
marginal piliferous pits, of which the 
first two features are characters of 
P. undulatus, the last is the feature of 
P. depressus, so | have combined the 
two species. A second plate does not 
Fig. 133. Two movable 
plates—natural size, have the marginal, pits but is.otherwise 
the same. I expect considerable variation in the pattern 
on plates from different regions of the carapace. 
Peltephilus protervus Ameghino 
P. protervus, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 509. 
This species, of which we found no representative, is 
very large. The plates of the type have two pits on the 
