Hylaeamys megacephalus,[Note 1] with which T. talamancae was synonymized for some decades, is similar in body size, but is not known to overlap with T. talamancae in range.
Later in the same year, he, together with Alexandre Percequillo and Robert Voss, named ten new genera for species previously placed in Oryzomys, including Transandinomys, which has Oryzomys talamancae (now Transandinomys talamancae) as its type species.
They are about as large, but in T. talamancae the tail is longer and the hindfeet shorter.
Each of the upper molars has three roots (two at the labial, or outer, side and one at the lingual, or inner, side) and each of the lowers has two (one at the front and one at the back); T. talamancae lacks the additional small roots that are present in various other oryzomyines, including species of Euryoryzomys, Nephelomys, and Handleyomys.
He placed both panamensis and carrikeri as synonyms of Oryzomys talamancae and mentioned O. mollipilosus and O. medius as closely related species.
In 2006, Weksler and colleagues noted tail coloration as a difference between the two species of Transandinomys (bicolored in T. talamancae and unicolored in T. bolivaris), but in their 1998 study, Musser and colleagues could not find differences in tail coloration between their Panamanian samples of the two species.
It is smaller and darker, but young adult T. talamancae are similar in color to adult H. alfaroi and often misidentified.
O. talamancae was the only member of its own species group, which Goldman regarded as closest to Oryzomys bombycinus (=Transandinomys bolivaris).
