146 Tlic American Geologist. September, i899 
Jurassic to the Post-Pliocene. Unquestionably, the one dis- 
covery which is always foremost in men's minds in a consid- 
eration of his work is the determination of an extinct order of 
birds possessed with teeth. The study of the dinosaurs and 
toothed birds showed that one by one characters considered 
as avian were likewise present in reptiles, and that many rep- 
tilian characters were present in these primitive birds; so that 
at the end there did not seem much else besides feathers to 
distinguish them. Marsh's investigation of fossil birds led to 
the publication, in 1880, of his first monograph, "Odontor- 
nithes: a Monograph on the Extinct Toothed Birds of North 
America." In this volume, he carefully figured and described 
all the known types, and presented complete restorations of the 
two leading genera Hesperornis and IcJithyor/iis. He con- 
cluded that birds most nearly resemble some of the small 
dinosaurs from the American Jurassic, and that both classes 
originated at least as far back as the Trias or late Paleozoic, 
in some sauropsid type. 
A discovery which rivalled that of the toothed birds, al- 
though not so wholly his, was the genealogy of the horse. 
Huxley and Kovalevski traced the equine branch through the 
Pliocene to the Upper Miocene in Europe, but the true and 
remote ancestry remained unsolved until the American types 
were described by Marsh. He showed that a primitive and 
diminutive polydactyl horse existed in the Lower Eocene, and 
that from this type, by gradual and progressive change through 
successive horizons of the Eocene, Miocene, and Pliocene, 
there had been evolved all the intermediate stages leading to 
the modern horse. 
Next in importance and interest should be noticed the series 
of papers culminating in the monograph of the Dinocerata, 
issued in 1886 by the United States geological survey. His 
work in other groups of mammals is scattered through a large 
number of separate papers, and contributions were made to 
every known order. The Tillodontia comprise one of the 
most remarkable of the types. Among others are the first 
remains of fossil primates, Cheiroptera, and Marsupialia, 
known from North America. The Brontotheridae and Cory- 
phodontia received considerable attention. A monograph 
had been begun on the former, and restorations of a typical 
genus of each were published. 
