1894.] 



NATURAL SCIENCKS OF PHILADELPHIA. 



229 



Molar crowns not pri.sniatit' ; loops closely appressed ; enamel 

 folds of unequal thickness, and rising at the free ends to form half 

 tubercles; antorbital vacuities with a blunt spine projecting forward 

 from top of outer side ; palate not excavated between posterior 



molars Sic/inodnn (fig. Id), Scapteromys, Holochilus(?), and 



other genera. 



In order to render the present account of these highl}^ interesting 

 rodents as complete and useful as possible, the genera Pty.'<mphori(s 

 and Trefomys are redefined, tlie new and closely allied genus 

 Hodomys is characterized (with reference to the moi-e specialized 

 genera Xenomys and Xentomu), and descriptions of all the known 

 species are added. 



Genus PTYSSOPHORUS Ameghino [Fossil], 



(Fig. 1, a, b, and c.) 



Plyssop/iojiis Ameghino, Mamiferos Fosiles Eepublica Argentina, 1889, 111, I12, 

 and pi. iv, figs. 1-lr. Tjqie Ptyssof^hoitis clci^aiis Ainegli.. IVom the Argentine 

 Republic. 



Crown of m ,; shaped like the letter S placed lengthwise of jaw; 

 projecting part of lower incisor nearly straight, slender, and formino- 



^li» 



Fig. 1. a, d, i\ Ph'ssop/iorus elegans (from Ameghino). 



a. Right ramus of mandible, outer side. 



b. Same, inner side (enlarged). 



c. ( "rowns of rigli t lower molars ( enlarged ). 



(/. Sigmodon hispidus, crowns of right lower molars. 

 e. Hodomys allcni, crowns of right lower molars. 



