Prof. 0. O. Marsh—Restoration of Brontops. 99 
that species is intercalated at the inner angles of the four plates 
in front of the median occipital. Fig. 3 represents the broad rounded 
operculum. The scales seem to be very thin, so much so that the 
internal skeleton is often seen through them, the axis being noto- 
chordal, and well-developed ribs being present. The leading feature 
of the species is seen in the form of the second dorsal fin, which 
has its base proportionally nearly twice as long as in D. Valenciennesit, 
while the shape of the fin, instead of being triangular-acuminate, is 
broad and oblong-rounded, the posterior rays being nearly as long 
as the anterior ones. Bota pectoral and ventral fins are as in 
D. Valenciennesti “ arehipterygian” in their conformation, the 
pectoral being represented in Fig. 4. The first dorsal and the anal 
are narrow-lanceolate, the heterocercal caudal is triangular, and not 
bifurcated. 
EXPLANATION OF PLATE II. 
Fig. 1. Dipterus macropterus, Traquair, natural size. From the Lower Old 
Red Sandstone, John O’Groats, Caithness. 
Fig. 2. Outline of posterior cranial plates, from another specimen. 
Fig. 3. Operculum. 
Fig. 4. Pectoral fin and shoulder girdle. 
IJ.—Restoration or Browtors RoBuUsTUS, FROM THE Miocene 
or AMERICA! 
By Professor O. C. Marsu, Ph.D., LL.D., F.G.S. 
(PLATE IV.) 
HE largest mammals of the American Miocene were the huge 
Brontotheride, which lived in great numbers on the eastern 
flanks of the Rocky Mountains, and were entombed in the fresh- 
water lakes of that region. They were larger than the Dinocerata 
of the Hocene, and nearly equalled in size the existing Hlephant. 
They constitute a distinct family of Perissodactyles, and were more 
nearly allied to the Rhinoceros than to any other living forms. 
The deposits in which their remains are found have been called 
by the author the Brontotherium beds. They form a well-marked 
horizon at the base of the Miocene. These deposits are several 
hundred feet in thickness, and may be separated into different sub- 
divisions, each marked by distinct genera or species of these gigantic 
mammals. 
The author has made extensive explorations of these Miocene 
lake-basins, and has secured the remains of several hundred 
individuals of the Brontotheride, which will be fully described in 
a monograph, now well advanced towards completion, to be published 
by the United States Geological Survey. ‘The atlas of sixty litho- 
graphic plates is already printed, and the author submitted a copy 
to the Section. The last plate of this volume is devoted to a restora- 
tion of Brontops robustus, one-seventh natural size, and a diagram 
enlarged from this plate to natural size was also exhibited.? 
1 Abstract of a paper read before Section D of the British Association for the 
Advancement of Science, at the Bath meeting, Sept. 7th, 1888. 
% The present Plate (IV.), one twenty-fourth natural size, shows a reduced copy 
of the same restoration, 
