1897.] Geology and Paleontology. 57 
This animal differs, however, from the contemporary horses in the 
prominent median cusps (mesostyle) of the superior molars, and the 
asymmetry of the outer wall (ectoloph) caused by the prominent par- 
astyle. This forbids our placing it with the true line of horses. The 
molars and the pes resemble those of the imperfectly known Triplopus 
amarorum & species whose relations to Triplopus the writer has always 
doubted. 
It appears possible that we have here another side line of perisso- 
dactyls, related to the horses. —Hrnry F. OSBORN. 
Development of the Foot in the Paleosyopinz.—The fol- 
lowing observations are based on the specimens contained in the col- 
lections of the American Museum of Natural History of New York. 
The Lower Eocene member of the group, Paleosyops borealis Cope, 
was rather slenderly built, with comparatively long toes, well sepa- 
rated. With the great increase in size in the Middle and Upper 
Eocene came a corresponding change in foot structure. Two types 
developed, one with short broad foot, the toe bones short, stout and 
widely spreading ; the other with longer and rather stilted foot, the 
metapodials long, but set close together. The former type is that of 
Palseosyops, and is correlated with a short wide head and general stout 
heavy build. The latter is the Telmatotherium type, and is associated 
with long heads and probably much more slender form. The species 
ean be conveniently distinguished by two characters in the astragalus, 
viz., the length and thickness of the neck, and the shape and relations 
of the sustentacular facet. 
In an astragalus referred to P. borealis, the neck is moderately 
long and not thickened at the base. The sustentacular is a rather 
long oval, and scarcely separated from the distal (cuboid and navicu- 
lar) facets. In our specimens of P. laticeps Marsh, the characters are 
much the same asthe above. P. paludosus Leidy, and P. ultimus’ 
Osborn, have a broad, short-necked astragalus, the sustentacular facet, 
in the former species at least, being short-oval and generally (but not 
always) well separated from the distal facets.’ The other (Telmato- 
therid) line shows a slight lengthening and considerable thickening of 
the neck, and a change in the shape of the sustentacular facet, either 
to an extremely long oval, separate from the distal facets, as seen in a 
specimen referred to T. hyognathum Scott & Osborn, or else a long 
2 Species not yet described. 
3 It is not so, cae in the Princeton specimen described by Earle in his 
Memoir on Paleosy 
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