When the Petaluma horse, Neohipparion gidleyi Merriam, was described in 1915 knowledge of the sequence of equine species had not progressed far enough to permit age or stage refinement in correlation. Three genera are now recognized among the hipparion-like horses that were supposedly indicative of lower Pliocene age in 1915. Furthermore as certain phyletic lines are recognized in each of these genera, it is possible to correlate early or late divisions of the provincial ages now used by vertebrate paleontologists.

There are two distinct species groups listed under the genus Neohipparion. The occidentale group is of Clarendonian age (early Pliocene) with the possibility of rare representation in the earliest Hemphillian (middle Pliocene). The eurystyle group is composed of Hemphillian species. N. gidleyi is clearly referable to the second group and its characters show that it is one of the most advanced species in the group.

Recent revaluations of the plants (Axelrod, 1944) and of the mollusks (Durham, oral communication) show that they also are indicative of middle Pliocene age for the formation.

Some of the most recent papers on the geology of the San Francisco Bay region (Morris and Bailey, 1935; Weaver, 1949; Bowen and Crippen, 1951; Taliaferro, 1951) have not used the evidence from the fossil vertebrates in the Petaluma formation but have continued to correlate the Petaluma with the Siesta or the Orinda (Clarendonian) of the Berkeley Hills. It should be recalled that no species of the eurystyle group has been found in the Siesta and Orinda or their equivalents north of the Tehachapi Mountains and west of the Sierra Nevada. If N. gidleyiis not a later Hemphillian (middle Pliocene) species (Stirton, 1939, 1940, 1951), then our concepts of the evolution in later Cenozoic Equidae are erroneous.