226 PliOCEEDINGS OF THE ACADEMY OF [1894. 



vidual; that of the latter is based oii a fragment of the niaxillarv 

 containing the upper molar series. At first, I was inclined to regard 

 Ameghino's specimens of Ptxjssophorns (fig. le) and Tretomys (fig. 2) 

 as the lower and upper jaws of the same animal, and to look upon 

 my ' Neotoma' alleiii (fig. le) as congeneric therewith.^ But sub- 

 sequent study has convinced me that FfysiiophorKS and Tretomy)< are 

 probably distinct, though closely related genera, and that the living 

 species formerly described as N. alleni, together with the new siiecies 

 here named vetulm, represent a third genus of the same group. For 

 this genus I propose the name Hodomys. Hodomys is a more recent 

 type than Plyssophorus, less specialized than Xenomys, and more 

 ancient than Neotoma. 



So far as dental characters go, the group of genera under con- 

 sideration (Ptyssophorus, Tretomys, Hodomys, Xenomy.% and Neotoma) 

 presents nearly every important step in the evolution of the modern 

 genus Neotoma from the Cricetine series. Sigmodoii seems to be the 

 connecting link that bridges the gap between the tuberculate toothed 

 Murine subfamily Cricetinse and the flat topped prismatic crowned 

 Neotominie, by which name it seems proper to designate the new 

 subfamily, comprising the 5 genera above enumerated.' Sigmodon 

 should be looked upon as an ancestral rather than a contemporary 

 type. It is almost on the dividing line between the tuberculate and 

 flat crowned groups, and is probably on or near the trunk line 

 along which the Neotomime branched off from the tuberculate series. 

 It is evidently an ancient type, dating back to the Pliocene at least,'' 

 since which period it has not undergone very marked changes. In 

 early life Sigmodon has the outer ends of the loops elevated, forming 

 half tuberculate grinders, much as in the American Cricetines, but 

 the projecting loops are soon worn down, leaving flat grinding sur- 

 faces (fig. !(/). The loops, however, remain closely appressed or 

 even soldered together, never standing out freely as in Neotoma and 

 Arvieola. 



* While of this mind I fstated, in u rccont paper on the geiuis ^Veo/oii/a. that 

 the species of Wddd Kats havinit? the ciown of tlic last lower molar shaped like 

 the letter S. were transferred to the iicnna J'/ ]'sso/>/ior/is of Ameiihino ( I'roe. 

 Biol. Sou. Wash., IX, July 2, 1894, 117. 



■' It is i)r()hal)le that several other animals deserihed by Ameghino belong to 

 the Neolo»iincc — such as Boilirioniys and some of the species rel'erred to the 

 genera Oxymiclerus, Holochilus^'AnA ' Habhrotrix'' {=^Abrolhrix). 



^ Ameghino has figured an nndoul)ted Sigmodon from the Pam])ean Pliocene 

 (Mamif. Fos. .\rgeiitinos, pi. 4, fig. lit?), and has referred the same to Holnchilus 

 ~i'u/piiins iJclif. 



