In an Upper Miocene assemblage of mammals recently brought together, occur remains of numerous individuals referable to the genus Merycodus. The collection represents several seasons of palaeontological field work by parties from the California Institute of Technology in the vicinity of Tonopah, Nevada.
The specimens of Merycodus exhibit characters that are not common to those of species described from other localities in the Great Basin Province or from West Coast Miocene and Pliocene deposits. In some respects, the new Nevada forms combine characters seen in M. furcatus and M. necatus, found in deposits of similar age. Differences of structure among the materials represented indicate the presence of two distinct species of Tonopah merycodonts.
In a study of the new types, an analysis of the morphologic characters of the genus emphasizes the essentially constant premolar tooth pattern for Merycodus. As may be expected, greater diversity occurs among merycodont horn-core structures, particularly as to shape and size. However, the supraorbital position of the horn-cores is apparently constant within specific groups.
In the present comparative studies, I am particularly indebted to Dr. Charles L. Camp and to Mr. R. A. Stirton of the Museum of Palaeontology, University of California, for the loan of Merycodus material. Mr. John L. Ridgway has prepared the drawings and has arranged the plates
