Vol. XI, pp. 211-11^ December 17, 1897 



PROCEEDINGS 



BIOLOGICAL SOCIETY OF WASHINGTON 



NELSONIA NEOTOMODON, A NEW GENUS AND SPE- 

 CIES OF MURINE RODENT FROM MEXICO. 



BY C. HART MERRIAM. 



The collection of mammals made in Mexico by Mr. E. W. 

 Nelson still contains many novelties. Recently, in looking at 

 the skull of a large White-footed Mouse from the mountains of 

 Zaeatecas, supposed to be a tyj)ical Peromyscus, I was startled 

 to find that it had the flat-topped prismatic teeth of a Wood 

 Rat, Xeoioma. Closer examination of the dentition showed that 

 while the 1st and 2d upper and 1st lower molars agree essen- 

 tially with those of Neotoma, the 3d upper and 2d and 3d lower 

 differ so materially that it is necessary to erect a new genus for 

 the animal's reception. The skull also is peculiar and unlike 

 either Ncotomd or Peromyscus. The most important difference is 

 in the antorbital slit, which does not notch the upper surface of 

 the maxillary root of the zygoma. Another character is tlie 

 production of the inferior angle of the antorbital slit to form a 

 distinct process. The tail is large and Itlunt, much as in Neo- 

 toma — not tapering to a slender point as in Peromyscus. 



The new genus may be defined as follows : 



Nelsonia gen. nov. 



Type. — Nelsonia neotomodon sp. nov., from Plateado, Zaeatecas, Mexico. 



Diagnosis. — Cranial characters: Skull in general resembling that of a 

 large Pei'omyscas but flatter ; zygomata heavier, less depressed, and more 

 spreading anteriorly ; antorbital slits relatively narrow and only faintly 

 notching upper surface of maxillary root of zygoma ; inferior angle of 

 antorbital slit thickened and protruding foi'ward and outward as a dis- 

 tinct process ; audital buUse sub-conical as in Pfromi/scnx and Hodurnys, 



C2-Bi()i.. Sor. Wash., Vol,. XI, 1897 (277) 



