[Illustration: FIG. 1. _Proechimys dimidiatus_ (Günther). Live female on left and male on right. ×1/2. From Tingua, Nova Iguassú, Rio de Janeiro, Brazil. Photographed in spring (August or September) of 1942 by author.] INTRODUCTION The spiny-rats included in the genus _Proechimys_ are common in almost every forest of South America above the Tropic of Capricorn, and in Central America northward to approximately 12° N, in Nicaragua. In size and proportions they are similar to the brown rat _Rattus norvegicus_ but actually they belong to a very different suborder of rodents--the Hystricomorpha. The hystricomorphs are represented in South America by a large variety of animals, of which capybaras, agoutis and cavies are common representatives. The pelage of the spiny-rats has a large number of flattened, spinelike hairs, especially on the back. The color ranges through different tints and shades of reddish-brown more or less evenly distributed on the upper parts; the underparts are usually pure white, sharply contrasting with the brown color above. The tail is bicolored, brown above and white below. The spiny-rats live in forests of different types, generally in the proximity of water. Shelter is usually procured under boulders, stumps or masses of roots. The reproductive rate is low; on the average, there are only two young per litter and only two litters per year. Sixty-odd names have been given to species and subspecies of _Proechimys_ in the last hundred and fifty years and no serious revision of the taxonomy of the genus was undertaken in the last century. The purpose of the present work is to provide means of understanding species and subspecies within the genus and to describe the different kinds known to occur within the confines of Brazil. METHODS AND TERMINOLOGY PELAGE.--It was found advisable to use a standardized nomenclature for hairs. The names here proposed are a choice of those used in the literature, with the suffix "_form_" as an element of uniformity. I feel that it would be advantageous if everyone adopted a similar universal system in mammalogy. The names listed below are used as nouns and are considered as English versions which could easily be adapted to different languages. These names may be complemented with adjectives as needed. Examples are lanceolate aristiforms, spinous aristiforms, and woolly setiforms. _Aristiforms:_ The most conspicuously developed hairs in a three-layered pelage or the corresponding hairs in a simpler pelage. Names previously used for these hairs are: guard hair, leithaar and jarre. _Setiforms:_ Common to all species and most numerous throughout the pelage; second in conspicuousness, being the dominant hairs in the middle layer. Synonyms are: over hairs, grannenhaare and soies.