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 

 CT^r-} g^™^ marginal piliferous pits, of which the 

 Ur.H ^v r -* v >>'^/ first two features are characters of 

 :?'*; '"'■''"•".J P- undulatus, the last is the feature of 

 ( ^iij P- depressus, so I have combined the 



~ .. two species. A second plate does not 



Fig. ijj. Two movable l i 



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 



