Molarization of the premolars in the Perissodactyla.

SUMMARY. Molarization of the premolars in perissodactyls is reviewed in the light of that of the milk‐molars. The premolar patterns are closely related to those of the milk‐molars, the closest resemblance being shown, in most forms, to the milk‐molar that the premolar replaces. In a few Eocene forms, however, the premolars seem rather to resemble milk‐molars placed further forward in the series. In primitive perissodactyls, each premolar differs from the corresponding milk‐molar in a number of ways, the most striking of which are the weak development of the talonid on the lower tooth and of the middle part of the crown on the upper tooth. The paracone and metacone of the upper premolar are more closely united than in the corresponding milk‐molar, and the protocone and hypocone are represented by a single cusp, the deuterocone. This condition is termed hyposphyric. In the course of evolution the hyposphyric condition gives place to an isosphyric condition in which each premolar closely resembles in pattern the corresponding milk‐molar. If the milk‐molar is not yet molariform, further molarization of the milk‐molar and premolar take place together. The lingual cusp (deuterocone) must be considered as homologous with the protocone and hypocone taken together. When on the milk‐molar the hypocone is larger than the protocone, the deuterocone of the premolar corresponds mainly to the hypocone and is converted into the hypocone as isosphyry increases, the protocone developing from its anterior crest. When the protocone and hypocone of the milk‐molar are equal. the deuterocone divides equally. On P4 the development of the hypocone is retarded, and the deuterocone corresponds mainly to the protocone. The hypocone when it first develops on P4 is smaller, lower and less lingual than the protocone. In most rhinocerotoids and some tapiroids a lesser degree of retardation of the hypocone occurs on P3. Molarization of the premolars is thus a complex of a number of processes which can be detected in all perissodactyls. Differences in the method of molarization from group to group are due to the lack of correlation between the processes involved. Differences in the course of evolution followed by adjacent premolars, as in Eocene Equidae and Oligocene Rhinocerotoidea. are to be explained in the same way. Premolar occlusion is closely related to that of the milk‐molars, corresponding cusps having similar occlusal relations in the two series.