544 PROCEEDmCS of the national IVIUSEUM voL.no 



Oryzomys laticeps * is a terrestrial species ranging coextensively 

 with Oecomys. Its resemblance to Oryzomys concolor is superficial 

 and restricted to bod}^ size and, in some cases, to color and texture of 

 pelage. In laticeps, however, the tail averages shorter, the hind foot 

 (fig. 4:d) longer, the ear larger. The outer toes of the hind foot of 

 0. laticeps are shorter than in all others wath which comparisons have 

 been made. A dark juvenal color-pelage phase and a distinct sub- 

 adult color-pelage phase are well marked and more consistently 

 present in laticeps than in other oryzomyines. The skull (e of pis. 

 8-11) of 0. laticeps is comparatively longer than in concolor, the 

 supraorbital region narrower, the sides ridged or beaded but not as 

 widely divergent; rostrum with a more decided downward flexion; 

 anterior zygomatic plate projecting well forward; anteroposterior 

 width of zygomatic plate often more than length of M^~^; incisive 

 foramina extremely short, generally less than 60 percent of length of 

 diastema, their posterior borders separated from anterior plane of 

 first molars by a distance approximately equal to length of first molar ; 

 sphenopalatine vacuities absent or small as in concolor; bones of 

 posteropalatal region more cancellate with the excrescences or cornu- 

 ate processes noted in concolor generally present and, usually, more 

 developed; internal folds of molars (pi. 12e) usually absent; second 

 secondary fold absent in M^; second secondary folds of lower molars 

 greatly reduced, sometimes coalesced with major fold of opposite side. 



Color and pelage: The range of variation in color and character 

 of pelage in Oryzomys concolor is wider than in 0. hicolor. Populations 

 of dry localities are distinctly paler than those of humid areas. The 

 pelage of high altitude forms is invariably longer and thicker than that 

 of their relatives of lower levels. As in 0. hicolor, there are bright and 

 dark color phases. The tone of the bright phase in a darldy colored 

 population generally equals that of the dark phase of a pale colored 

 population. In concolor, individual molt from one color phase to the 

 other is obvious in a large proportion of the specimens. 



Pelage of nestlings is short, fine, and colored like that of adults. 

 This is superseded by a longer dark gray or dark brown coat distinctly 

 darker than that of adults. 



The Subspecies 



Five geographic races are recognized. The nominate subspecies, 

 Oryzomys concolor concolor Wagner, occupies the greatest part of the 



* Specific synonyms of Oryzomys laticeps include bolivans 3. A. Allen, boliviae Thomas, caracolus Thomas, 

 casteneus Thomas, goeldi Thomas, legatus Thomas. macconnelU Thomas, magdalenae J. A. Allen, medius 

 Robiason and Lyon, modestus J. A. Allen, mollipilosus J. A. Allen, oniscus Thomas, perenensis J. A. Allen, 

 rivularis J. A. Allen, taltator Winge, syhaticus Thomas, talamancae 3. A. Allen, velutinus J. A. Allen, 

 and a few others, yfus capita Olfers (1818), based on the "rat & crosse t6te" of Azara, is undoubtedly the 

 same species and antedates Mus laticeps Lund (1841). Mus cephalotes Desmarest (1819) is another name 

 for Azara's "rat ^ grosse tSte." 



