1894.] NATI'RAL SCIENCES OF PHILADELPHIA. 231 



specific weight is the length of the enamel folds, a character appa- 

 rently due to anticiuity, representing an earlier and more primitive 

 stage in the evolution of the enamel pattern. In Ameghino's single 

 specimen the re-entrant angles or grooves between the folds extend 

 vertically from crown to alveolus, as in the young of Neotoma and 

 allied types. Whether they continue below the alveolus to the very 

 root of the tooth as in the Arvicolines, we are not informed; but the 

 characters of the jaw and molar crowns indicate that they do not. 

 The specimen is apparently immature and the teeth are probably 

 rooted or semi-rooted. 



Ptyssophorus elegans Aiiu'fihiiio. (Fig. 1, a, h, and c.) 



Ptvssophorus damans Anieghino, Mamiferos Fosiles Repub. Argentina, 1889, 

 111, 112. and pi. 4, tigs. 1, \c. 



Based on a fossil ramus of the mandible (right side) from the 

 Pampean Pliocene deposits near Villa de Lujan, Province of Buenos 

 Ay res. 



Specific characters. — Most of the characters have been given above 

 in the generic diagnosis and need not be repeated. The anterior 

 loop of m I has a lobe directed forward (see fig. 1c). Ameghino 

 states that the alveolar border is higher on the outer than inner side 

 of the jaw, and that m i is nearly as large as m _> and 3 together. 

 He gives the following measurements: length of molar series on 

 crowns, 5 mm.; on alveolus, (> ; distance from incisor to m 1, 5; 

 height of ramus at m ■{, 5; distance from front of incisor to back 

 of last molar, 12. The incisor is shoi't, but this may be an indi- 

 vidual peculiarity. The hinder part of the mandible is broken off, 

 so that the form of the angle and condylar ramus can only be in- 

 ferred from allied forms. 



Ameghino's figures are here reproduced (fig. 1, a, b, and e). 



Genus TRETOMYS Ameghino [Fossil]. 



(Fig. 2.) 



Tietoniys Anicgliinci, Main. Fos. Rcpiil). Argent.. 18.S9, 110. pi. 4. figs. 1(5 and 16(7. 

 Type Trrloiiiys ala7'ii.< Ameghino. IVoni Pampean I'lioi-ene, Argentine Repul)lic. 



Generic characters (based on Ameghino's figures and description 

 of part of maxillary bone including upper molar series, maxillary 

 root of zygoma, and upper incisor). — Number of enamel folds as 



