380 SECTIONAL TRANSACTIONS.—C. 
2. Prof. Grorcr D. LauprrBack.—Tectonic Geology of the Tsingling 
Shan, China. 
3. Dr. W. D. Marruew.—A New Link in the Ancestry of the Horse. 
The series of American Tertiary ancestors of the horse is one of the classic 
examples of evolution provided by the fossil record, and the most complete 
and convincing among the mammals. Nevertheless it is well recognised by 
those who have made a special study of it that, while the broader lines of 
descent are beyond reasonable question, there are definite gaps between some 
of the successive stages, and many minor problems as to the details of phylogeny. 
It has been confidently believed that these gaps would be filled by the dis- 
covery of fossil Equide in the intervening strata heretofore barren, and from 
time to time such discoveries have been made and the predictions as to their 
character always more or less precisely verified. 
The most serious gap that exists at present is between the Upper Eocene 
Epihippus and the Lower Oligocene Mesohippus, and this gap still remains. 
I anticipate that it will be at least partly filled by systematic search in certain 
western and northern outliers of the White River Oligocene, which contain an 
older fauna than the main exposures, but are very little known. 
A second gap existed twenty years ago between Miohippus of the Upper 
Oligocene and Merychippus of the later Miocene, but this has been filled by 
the discovery in 1905-1916 of numerous species of Parahippus in the intervening 
Lower Miocene strata, so that it is now not easy to draw the lines of demarcation 
above and below Parahippus. 
A third gap existed between the Lower Pliocene Hipparion-Protohippus- 
Pliohippus group and the Pleistocene and uppermost Pliocene /quus, the modern 
type of horse. The importance of this gap has been somewhat overestimated by 
some European authorities, whose estimate was probably based upon comparison 
of the European species of Hipparion, with the true Lquus; it is, in fact, a 
rather small gap if comparisons be made with the various American species of 
these three genera. Some of these, however, were very incompletely known. 
The Middle Pliocene species referred to Pliohippus, from the Blanco formation 
of Texas and the Etchegoin of California, were intermediate in geological age, 
and in characters as far as known ; but all that was known of them was a couple 
of teeth from the Blanco and three or four from the Etchegoin. 
Mr. Childs Frick, who has been making a special study of the Equida, 
had urged upon me for some time the importance of further work in the 
Blanco formation, and provided the funds for such field work. This was taken 
up this spring with very satisfactory results. We were fortunate to secure, 
among other things, a nearly complete skeleton, lacking only the skull, and a 
second partial skeleton with well-preserved skull, of the large so-called Plio- 
hippus from the Blanco. The first was found by myself, the second by my 
assistant, Mr. George Simpson. 
It proves to be a very interesting type, intermediate between the typical 
Pliohippus of the Lower Pliocene and Hquus of the Lower Pleistocene, so far 
as the characters were observed in the field. 
It is of the size and limb-proportions of the smaller species of Equus, much 
larger and more robust than true Pliohippus. The teeth are most like Plio- 
hippus, but longer-crowned and less curved, with heavier mesostyle and larger 
and more nearly isolated protocone. The skull has the elongate proportions of 
#quus in contrast to the shorter skull of typical Pliohippus and earlier Equids. 
It retains in the fore foot a tiny vestigial nodule representing the first digit, 
progressively reduced in the earlier stages of Equide, completely lost in Hquus. 
The splints are little more than half the length of the cannon bone, nearly or 
quite as much reduced as in Hquus, while in true Pliohippus the splints are 
almost as long as the cannon bone, and it is not certain that the lateral phalanges 
had entirely disappeared except in one unusually progressive species. 
Many other characters will doubtless appear in the course of preparation and 
study of the two skeletons. 
The intermediate character of the California species has been recognised by 
Dr. Merriam from the few teeth which he had to study. The discovery of 
