4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 139 



Size, flatness, and ridging of the skull increase with advancing age 

 in Hoplomys and Proechimys. Tooth wear appears to be a reliable 

 criterion of age. Full adult pelage usually is attained after M3 appears 

 and before it becomes functional. Only juvenile and adult pelages have 

 been distinguished. Specimens in which all cheek teeth are functional 

 are considered to be adults. Generally, the largest, flattest, most 

 heavily ridged skulls have the most worn teeth. Apparently these 

 rodents continue to grow after all teeth are functional. Thus, there 

 is considerable size spread among adult skulls. For this reason only 

 maximum and minimum figures are given in the table of measurements 

 (p. 6). 



Body sizes appear to be uniform throughout the mainland range 

 of the species, but larger on Escudo de Veraguas. The skull is narrow 

 in the south — Ecuador, Colombia, and Darien — somewhat broader 

 in central Panama, Costa Rica, and Nicaragua, and broadest of all 

 on Escudo de Veraguas. Likewise the nasals and cheek teeth are 

 smaller in the southern populations. Size of the auditory bullae 

 increases northward from Ecuador to Nicaragua, but the bullae are 

 largest and most inflated anterolaterally in the Escudo specimen. 

 Several features of the zygomatic arches vary geographically. The 

 maxillary roots of the zygomata flare less widely and less perpendicu- 

 larly from the longitudinal axis of the skull (so that the zygomata are 

 more convergent anteriorly) from the Canal Zone southward than 

 they do in the north. From Cerro Azul southward the maxillary roots 

 tend to flare up, away from the ventral plane of the skull, rather than 

 paralleling that plane as they do in the north. The jugal has a hooklike 

 posteroventral process in most Canal Zone and Cerru Azul specimens, 

 but not in others. Most of the specimens from Ecuador, Colombia, 

 and Darien, and a smaller percentage of the central Panamanian 

 specimens have a small conical projection on the dorsal edge of the 

 zygoma at the jugal-squamosal suture. I failed to check this character 

 in the Costa Rican and Nicaraguan specimens. There is hardly a 

 trace of it in the Escudo individual. The nasals, broad and posteriorly 

 truncate in the island specimen, are usually narrower and posteriorly 

 acute in mainland populations. 



Among mainland populations of Hoplomys flatness and ridging of 

 the skulls of mature individuals are similar to these features in mature 

 individuals of Proechimys semispinosus. None of the available 

 Hoplomys or Proechimys closely approaches the Escudo specimen in 

 flatness or ridging, despite the fact that the island specimen, judged 

 by tooth wear, is a prime adult, not as old as many individuals with 



