Prozaedius Ameghino

Of this little genus, which is so strikingly like the living Zaedius, we found a carapace with ten rows of movable 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:

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

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 pits, the median lateral plates with rudimentary piliferous pits, and the dorsal ones with well marked posterior pits. In each succeeding row toward the rear, the plates are more distinctly ornamented and have larger posterior marginal pits. I have no marginal plates.

Fig. 132. Portion of carapace—natural size;
unshaded plates are from cast; a and
b plates from pelvic buckler.

The plates of the pelvic buckler do not overlap, are shorter, have a very distinct figure, and, in addition to the longitudinal furrows, have a couple of radial furrows on either side, which divide the plate into four or five areas ([see fig. 132 a and b]).

The caudal vertebrae are short and thick, indicating a short tail. I found no plates which would indicate a caudal shield, which coincides with the experience among the Santa Cruz specimens. The figures are to scale and give most of the measurements.

There are ten rows of movable plates, probably two to three rows lacking.

There are twenty + plates to a row.

A typical movable plate measures 17 mm. long by 6 mm. wide.

There were at least four rows in the pelvic buckler, probably eight as in the Santa Cruz.

A typical fixed plate measures 10 mm. long by 5 mm. wide.

Prozaedius tenuissimus Ameghino

P. tenuissimus Amegh., 1902, Bol. Acad. Nac. Cienc. Cordoba, t. 17, p. 66.

This species is characterized by Ameghino on account of its small size. The movable plates have two furrows which converge toward the front, and between which is a median crest. In the furrows are two rows of perforations. A movable plate measures 9 mm. long by 4 mm. wide.