_Villiforms:_ The smallest hairs in the three-layered pelage. Synonyms are: underfur, wollhaar and duvet. _Vibrissiforms:_ The vibrissae proper, or any typically sensory hair. TEETH.--The tritubercular nomenclature was abandoned because of overwhelming difficulties; more research on the Hystricomorpha is certainly needed before the tritubercular nomenclature can be applied with confidence. The following names are used for features of the molariform teeth: _Main fold:_ The inner or lingual fold in the upper molariform teeth and outer or labial fold in the lower molariform teeth. _Counterfold:_ Any outer or labial fold in the upper or any inner or lingual fold in the lower molariform teeth. For incisors Thomas (1921:141) is followed: _opisthodont_, _orthodont_ and _proodont_ depending on the angle between the exposed part of incisors and the ventral surface of the rostrum. The capital letters P and M designate premolars and molars, respectively, of the upper jaws; lower case letters p and m designate corresponding teeth in the lower jaws. MEASUREMENTS.--Measurements of skins were used only when provided by the collector. The length of the hind-foot is intended to be always _cum unguis_, but in a few instances it is impossible to be sure whether the collector included the nail. Length of tail was used only when the tail seemed not to be mutilated. Ear measurements taken by collectors are scarce. In spite of the apparent usefulness of length of ear, it was found to be inadvisable to take the measurement on the dry skins. The following measurements of the skull are used in the tables: _Greatest length:_ From the anteriormost part of the nasals to the posteriormost part of the supraoccipital. _Condylo-incisive length:_ From the anterior face of one incisor, at the alveolus, to the posteriormost part of the exoccipital condyle of the same side. _Zygomatic breadth:_ Maximum distance across zygomata in a plane perpendicular to longitudinal axis of the skull. _Length of nasals:_ Maximum length of one or both, whichever is the greater. _Interorbital constriction:_ Least width between the orbits on top of the skull. _Palatilar length:_ From the posterior face of an incisor, at the alveolus, to the nearest part of the posterior edge of the palatine bone. _Crown length of cheekteeth:_ From the anterior border of P4 to the posterior border of M3. In the accounts of species, measurements of aristiforms and setiforms are used. The hairs measured were taken from the middorsal region and outer thighs, and the measurements are means.