E. scotti, Rock Creek, Texas, is probably the best represented Middle Pleistocene horse (skulls and skeletons). Unfortunately the type (Gidley 1901, Plate XX) is a mounted skeleton (AMNH 10606) which apparently no one could study in detail. The skull AMNH 10612 figured by Gidley (1900, fig.3) is not the type skull. However, both well preserved and accessible skulls (AMNH 10612, New York and NMC 2381, Ottawa) as well as the skull published by Johnston (1937) are caballine according to Franck’s and Palatal indices.
The muzzle is moderately long and broad. The upper cheek teeth are plicated, with well developed plis caballins and long bilobated protocones. The lower cheek teeth have caballine or caballoid double knots and shallow ectoflexids.
At Ann Arbor there is a large skull broken in two fragments (V 46899a and b) from the Seymour Formation, Texas. It was wrongly referred to an E. grevyi-like form by Eisenmann and Baylac (2000) because of an error in the data. The referral to E. scotti by Hibbard and Dalquest (1966) is probably correct. Both indices may be estimated as caballine. The upper dentition of another specimen (V 46898) figured by these authors can also be easily referred to E. scotti.

References

Eisenmann, V. and Baylac, M. (2000) Extant and fossil Equus (Mammalia, Perissodactyla) skulls: a morphometric definition of the subgenus Equus. Zoologica Scripta 29(2), 89-100. 125.Extant and fossil Equus (Mammalia, Perissodactyla) skulls : a morphometric definition of the subgenus Equus.

Eisenmann V. (2006) Discriminating Equus skulls: The Franck’s Index and the new Palatal Index. 9th ICAZ Conference, Durham 2002. In: Equids in Time and Space, Marjan Mashkour ed., 13: 172-182, 13 figs. Oxbow Books. 148.Discriminating Equus skulls : The Franck’s Index and the new Palatal Index.

Gidley, J. W. (1900) A new species of Pleistocene horse from the Staked Plains of Texas. Bulletin of the American Museum of Natural History 13, 111-116.

Hay, O. P. (1913) Notes on some fossil horses with description of four new species. Proceedings of the United States National Museum 44(1969), 569-594.

Hibbard, C. W. and Dalquest, W. W. (1966) Fossils from the Seymour Formation of Knox and Baylor Counties, Texas, and their bearing on the Late Kansan climate of that region. Contributions from the Museum of Paleontology of the University of Michigan 21, 1-66.

Johnston C. S. (1937) Notes on the craniometry of E. scotti Gidley. Journal of Paleontology, XI(5), 459-461.