Nonomys gutzleri, sp. nov., is described on the basis of isolated teeth from the late Duchesnean or early Chadronian upper member of the Pomerado Conglomerate, San Diego County, California, U.S.A. It differs from Nonomys simplicidens (early to middle Chadronian of Wyoming and Texas) in several characters, including molars 10–13% larger, more conical metaconids and protoconids on ml and m2, and small entoconid on m3. Nonomys gutzleri further differs from N. simplicidens in the apparently derived characters of one or more distinct cuspulids on the labial cingulid of m1 and m2, weaker ectolophids on m1–3, and lack of preprotocristid and metalophulid on m1–3. These characters foreshadow the conditions in Diplolophus. An isolated m1 (SDSNH 72232) from the Pomerado Conglomerate represents an unnamed new taxon of nonomyine morphologically intermediate between Nonomys and Diplolophus. It differs from Nonomys in having m1 19–28% larger than N. gutzleri, a much larger metastylid, and, similar to the conditions in Diplolophus insolens, in lacking a distinct anteroconid and a complete ectolophid, but in possessing an arch-like ridge structure on the posterior flank of the protoconid. However, SDSNH 72232 is 59–68% smaller than the m1 in D. insolens, is anteroposteriorly more elongated and much lower-crowned than the m1 of D. insolens, and has a separate metaconid and metastylid. The derived similarities between Nonomys, SDSNH 72232, and Diplolophus support their inclusion in a redefined subfamily Nonomyinae, but this subfamily is not confidently assignable to either Muroidea or Dipodoidea, and is allocated only to Myodonta, incertae sedis.