200 THE DESEADO FORMATION OF PATAGONIA 
plates in place, parts of four rows of fixed plates from the 
pelvic buckler, and some caudal vertebrae. ‘The genus is 
distinguished by its thin plates, there being fourteen bands 
of movable plates, and eight rows in the pelvic buckler. 
The movable plates are narrow, each overlapped about a 
fourth of its length, and have a faint ornamentation, with no 
piliferous pits except on the posterior margin. ‘The fixed 
plates are similar, except that they are shorter, and have 
the ornamentation more accentuated, with radial grooves. 
Ameghino has described three species as follows: 
P. impressus, sculpture little accentuated, post. piliferic pits rudimentary, 
P. planus, sculpture more accentuated, post. piliferic pits lacking. 
P. tenuissimus, very small. 
In my specimen, the two anterior rows of movable plates 
lack the marginal piliferous pits, on the next two rows they 
are rudimentary (which is also true of the lateral plates 
even further back), while on the bulk of the movable plates 
and on those of the pelvic buckler there are two, three or 
four good-sized piliferous pits on the rear. I can therefore 
recognize but two species, P. impressus and P. tenuissimus. 
Prozaedius impressus Ameghino 
P. impressus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 508. 
P. planus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 509. 
Our specimen was found on the Chico del Chubut River, 
west of Puerto Visser, and preserves over two hundred 
plates, and eight caudal vertebrae. The anterior rows of 
plates of the carapace consist of thin plates overlapping 
about a fourth their length. Just behind the overlap, there 
is, on each, a group of small punctations, and the exposed 
part of the surface is divided by two shallow furrows, 
making three more or less equal ridges which die out toward 
the rear, leaving the posterior part of the plate plain. 
These most anterior plates are bent to one side and have 
no piliferous pits on the rear margin. ‘The plates of the 
third and fourth rows are not bent, and have the sculpture 
more distinct, the extreme lateral plates having no piliferous 
