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 



