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A charge is made on all overdue books. University of Illinois Library M32 7U,S ca 12 TUE PIRPARY Qe THe : oa ~ +i abst oe | M2 3 Ne FEB.20} JNIVERSITY OF ZOOLOGICAL SERIES atk OF FIELD MUSEUM OF NATURAL HISTORY Volume XX CHICAGO, DECEMBER 11, 1933 Pages 11-14 TWO NEW RODENTS FROM ARGENTINA By WILFRED H. OsGcoopD CURATOR, DEPARTMENT OF ZOOLOGY Among mammals collected by Colin C. Sanborn during the Marshall Field South American Expedition of 1926 is a bright- colored mouse from northeastern Argentina which obviously repre- sents an undescribed species. It is also of interest because it offers a further illustration of the increasing difficulty in applying the standards which have been set for generic divisions of neotropical rodents. In other words, it does not conform precisely to any one of several current genera and at the same time it does not differ from them sufficiently to justify the erection of a separate genus for its sole occupancy. Externally it suggests Oecomys, but its palate and dentition are those of Thomasomys, while the general shape of its skull inclines somewhat toward that of Rhipidomys. On the whole, it seems to differ least from Thomasomys and, therefore, may be placed in that genus. A slight subspecies of Hesperomys, likewise from Argentina, also may be described at this time. Thomasomys pictipes sp. nov. Type from Caraguatay, Rio Parana, 100 miles south of Rio Iguassu, Misiones, Argentina. No. 26,814 Field Museum of Natural History. Adult male. Collected Sept. 6, 1926, by C. C. Sanborn. Orig. No. 1,176. Diagnosis.—Probably allied to T. oenax, but decidedly smaller; tail slightly shorter than head and body; pelage softer than in Oecomys and Oryzomys, but not so long and full as in Andean species of Thomasomys; tail with a slight pencil, more than usual in Thomasomys, but less than in Rhipidomys; feet rather short and broad; color with Ochraceous-tawny prevailing, especially on the muzzle, rump, and feet; under parts bicolored except on throat; skull with short rostrum, rather wide braincase and zygomata scarcely compressed anteriorly; zygomatic plate nearly vertical, slightly No. 321 12 FreLD MusEUM oF NATURAL HISTORY—ZOOLOGY, VOL. XX visible from above; palate and teeth essentially as in Thomasomys and Rhipidomys. Color.—Upper parts Ochraceous-tawny, paler and much mixed with dusky on back and shoulders, brighter and clearer on rump and thighs; sides of muzzle clear Ochraceous-tawny, quite well defined and separated only by a slight median line of dusky above the rhinarium; under parts creamy, lightly washed with ochraceous, the hairs with broad, dark bases, except on chin and throat, where they are self-colored whitish buff; feet clear Ochraceous-tawny, the toes whitish; ears mixed dusky and tawny; tail bicolor except for the terminal half-inch, which is dusky all around. Skull —Skull rather short and broad; rostrum short but narrow; a marked interlacrymal depression behind nasals; zygomata well expanded and nearly parallel; zygomatic plate almost vertical, slightly visible from above; braincase broader than in Oecomys; frontals slightly wider than usual in Thomasomys, the supraorbital edges angled but not ridged; interparietal large; palatal slits short, not reaching to first molar; mesopterygoid fossa wide and parallel- sided, extending forward to middle of last molar, a single small foramen in the palatine on either side of its anterior boundary; cheekteeth rather small and narrow, the stylar elements well developed; first upper molar with protoconule and parastyle clearly divided by a deep cleft; first lower molar with protoconulid and paraconid also sharply divided. Measurements.—Type: total length 197; head and body 100; tail 97; hind foot 21; ear from notch (dry) 12. Skull of type: greatest length 26.3; basilar length 19.3; zygomatic breadth 14.4; greatest breadth of parietals 8; interparietal 10 x 3.8; interorbital constric- tion 4.5; breadth of braincase 12.3; nasals 9 x 3.2; length of zygomatic plate 2.5; diastema 6.5; postpalatilar length 9; palatine slits 4.3; upper cheekteeth 3.9. Remarks.—This species has about the size and proportions of Oecomys bicolor and, aside from its dark under parts and ‘‘pyrrhor- hine” face, it might pass on external characters for an Oecomys with rather soft pelage. Its skull and teeth, however, point to other relationships. Judging only from the description, it seems not improbable that the imperfectly known Thomasomys oenax of Rio Grande do Sul may be rather closely related. Although it is very much larger and longer-tailed, it is said also to have a bright-colored nose and to have a skull in which the zygomatic plate is slightly projected forward. It may be, therefore, that oenax and the present et eet “1933 NEw RoDENTS—Oscoop 13 species will prove connectant between Delomys and _ typical Thomasomys. When the name Delomys was proposed, it was urged in its favor that the species included was geographically removed and isolated from other members of Thomasomys. Later, in describing 7. oenax from the same region, however, Thomas ignores Delomys, and calmly states that oenax “is a Thomasomys, and represents a new species from an area where members of the latter genus had never previously been known.... A medium-sized species of the most typical cinereus section of the genus.’”’ By connecting oenax directly with cinereus in this way, the position of Delomys was greatly weakened not only on geographical but on morphological grounds, for cinereus (although the type of the genus) is a species in which the zygomatic plate is more projecting than usual in Thomasomys. The principal and practically the only cranial distinction of Delomys is its projecting infraorbital plate, so with two species like oenax and pictipes in proximity to it, there is little left to separate it except its mammary formula. It has eight mammae instead of six and its first upper molar has the anterior elements less deeply divided than in Thomasomys. These characters, as well as the projecting zygomatic plate and its rather harsh pelage, might be regarded as tendencies toward or away from Oryzomys. By this interpretation Delomys would be a form standing directly between Thomasomys and Oryzomys and it would be quite analogous to Microryzomys of the Andean region which stands in the same relative position although with a different combination of characters. In both cases, it seems to me, subgeneric rather than generic rank carries a better nomenclatural recognition of the facts. For the present, therefore, I should prefer to regard Delomys (probably with only one species) as a subgenus of Thomasomys. The species here described cannot be referred to Delomys, but if its mammary formula, now unknown, should prove to be eight instead of six, the situation would be further complicated. The external characters and dimensions of 7. pictipes do not differ widely from those ascribed to Rhagomys rufescens, the unique and imperfect type of which has never been duplicated after being received nearly fifty years ago from Rio Janeiro. The teeth of this type, in which stylar elements are practically lacking, are so unlike those of any well-known form that the animal’s relationships are very doubtful. Unless the skin and skull of this type have been improperly associated, therefore, it needs no consideration in the present connection. 14 FreELD MUSEUM OF NATURAL HISTORY—ZOOLOGY, VOL. XX Hesperomys bimaculatus bonariensis subsp. nov. Type from Torrecita, province of Buenos Aires, Argentina. No. 23,406 Field Museum of Natural History. Adult female. Collected May 31, 1923, by W. H. Osgood. Orig. No. 5,800. Diagnosis.—Similar to typical bimaculatus, but decidedly smaller and somewhat darker-colored. Color.—About as in bimaculatus; upper parts with marked con- centration of blackish forming a broad dark dorsal band from the forehead to the rump; conspicuous whitish postauricular spots; under parts white, the hairs on the throat wholly white, those elsewhere with slaty bases; tail bicolor. Skull.—Smaller throughout than in bimaculatus; palatine fora- mina markedly shorter; audital bullae smaller; teeth slightly smaller. Measurements——Type specimen and (in parentheses) an adult from Polanco, Uruguay: total length 187 (142); tail 48 (59); hind foot 15 (17.5). Skull of type: greatest length 20.5 (22.3); basilar length 16.3 (17.8); zygomatic breadth 11.8 (12.4); breadth of braincase 9.8 (10.2); nasals 7.2 (8.2); interorbital constrittion 3.3 (4); diastema 5.2 (5.7); palatine slits 5.2 (5.4); upper toothrow 3.3 (38.5). Remarks.—Specimens collected recently in Uruguay by Sanborn are doubtless representative of true bimaculatus as indicated by their small size, short tails, and conspicuous postauricular spots, these being the general characters which distinguish the species from all others of the region. In detail, however, they do not wholly agree with Waterhouse’s description and figure. They are darker in color than his figure and the hairs are not wholly white to the roots over the entire under parts, but only on the chin and throat. The extent of such white areas is often variable in other species and this discrepancy, therefore, may have no great significance. The skull figured by Waterhouse shows the nasals much narrowed behind, whereas in the skulls of the modern specimens the nasals are nearly parallel-sided and broad behind. If the skull of his type is still available, the accuracy of this figure may be tested. Mean- while, it seems necessary to judge the species by the modern speci- mens, which are from two localities, Polanco and Rio Cebollati, somewhat inland from the settlement of Maldonado, but within the territory traversed by Darwin and probably within what was in his time the province or department of Maldonado. Comparison of Uruguayan specimens with specimens from the other side of the Rio La Plata reveals pronounced distinctions in size. Therefore, the southern form is given the name bonariensis. | “HIN