suggestion which I wish to make. It may be of some use in explaining the phenomena, and it may not. I am disposed to attribute it to the tide, whose forced wave, sweeping every day over the successive ridges or through the successive archipelagoes which I have described, tore away the rocks and swept the fragments westward, rolling them over and over against one another until they were ground to pebbles and to sand. The tide-wave reaches the bottom of the deepest water, and is not a mere superficial current. Its motion is incessant, twice a day, and not occasional as that of the storm-wave. Finally, its direction in this region was westward, and it is a fact of no little significance, in this connection, that, so far as we can determine, all the material of these four sandstones has travelled westward. These considerations united induce me to believe that the tide-wave was the chief agent in their formation; that, rolling, as it did, every twelve hours from the East into the midland ocean of North America, through the successive archipelagoes or reefs which I have here attempted to describe, it acted as a grinding and transporting engine of transcendent power to fashion and to carry the sand and pebbles of which our great conglomerates consist.

There is nothing, so far as I am aware, in the rocks that is incompatible with these views. It is well known that the conglomerates are thicker and coarser in the East than in the West, and that there also pebbles of slate and other softer minerals are more abundant. Only the very hardest material-the quartzcould survive the wear and tear of so long and so rough a journey, and accordingly in the West this material constitutes. the whole mass of the rock.

One other point should be at least alluded to. Recent researches have rendered it probable that this great grinding engine, this tide-wave, was more powerful then than now; but on this I do not care to insist. . Sir Robert Ball's immense sixhundred-foot tide-wave must, I think, be relegated to a much earlier date. Yet the theory is entitled to whatever advantage may be derived from the greater tides of palæozoic time.

Should the suggestions here made and the views here advanced, regarding the origin and formation of the Conglomerates of Pennsylvania, prove to be of any value, they may indirectly bear on the moot question of the antiquity of the Atlantic

Ocean; for, if the transportation of the pebbles and sand was really due to the tide, it would indicate the existence of an Atlantic basin in pre-paleozoic days, from which the forced wave flowed over or through these successive reefs or ledges into the midland basin.

THE

THE PERISSODACTYLA.

BY E. D. COPE.

(Concluded from page 1007.)

`HE CHALICOTHERIIDÆ had numerous representatives during Eocene time, and a few species of Chalicotherium extended into Miocene time. The boundaries which separate the family from the Lophiodontidae on the one hand and the Menodontida on the other are not always easy to determine. From the former the symmetrically-developed external V's of the superior molars and the double V's of the inferior molars distinguish it; yet in Pachynolophus the anterior cingular cusp produces a part of the

FIG. 24. Lambdotherium popoagicum Cope, molar teeth, natural size; from Wind River Eocene of Wyoming. From Wortman, after Cope. Fig. a, second superior molar; b, last inferior molar. ae and pe, anterior and posterior external V's; y, intermediate external rib; x, anterior external angle; pi and ai, anterior and posterior internal tubercles; acc and pcc, anterior and posterior intermediate tubercles; h, heel.

asymmetry found in the Lophiodontidæ. The character of the double inner cusps of the superior premolars, which distinguishes the Menodontidæ, is only found in the last premolar in Diplacodon of the latter, while a trace of the additional cusp of this tooth is found in the Chalicotheroid Nestoritherium.

In using the following table it must be borne in mind that the structure of the feet has not been determined in several of the genera:

PLATE XXXII.

Menodus giganteus Leidy, one-twenty-fourth natural size; from White River (Oligocene) bed of Nebraska, Bulletin Mus. Compar. Zoology, Cambridge, Mass.

From Scott and Osborn, in

I. Internal cones of superior molars separate from external lobes.

A. External tubercles subconic, separated by a vertical external tubercle. Fourth inferior premolar like first true molar;................. Ectocium Cope. Third and fourth inferior premolars like the true molars;...Epihippus Marsh. AA. External tubercles of superior molars become V's, which are separated externally by a vertical ridge.

a. Incisors present.

B. No diastema in front of second inferior premolar. Second premolar without inner lobe; last molar with one

Second premolar with inner cone; last superior molar with an inner cone;.....

.Leurocephalus S. & O.

........Palæosyops Leidy.

Second premolar with inner cone; last superior molar with

two inner cones; .....................

BB. A diastema in front of second inferior premolar. Two inner cones of last superior molar;.........

aa. Incisors absent from both jaws.

Limnohyus Leidy.

Lambdotherium Cope.

Last superior molar with one internal cone;....... ................. Nestoritherium Kaup.

II. One or both internal cusps of superior molars united with the external lobes by

cross-crests.

a. External cusps of superior molars more or less conic.

An antero-external cingular cusp;.......

aa. External lobes of superior molars, inflected V's.

6. No crescentic inner lobes.

Intermediate lobes confluent ;.....

Pachynolophus Pomel.

a

.Chalicotherium Kaup.

FIG. 25. Ectocium osbornianum Cope, molars, natural size; from the Suessonian of Wyoming. Fig. a, superior molars; b, inferior molars. Original.

FIG. 26. Lambdotherium popoagicum Cope, lower jaw ramus, natural size; from Wind River Eocene of Wyoming. Original.

The phylogeny of this family is not difficult to read. Ectocium, if it be truly a member of it (the feet are unknown), is clearly the primitive genus, which is not far removed from Systemodon of the Lophidontidæ, in characters. The flattening of its external cusps produced the two external V's of the other genera, and this, without further modification, would give us Leurocephalus and Palæosyops, the former having the second superior premolars more simple than in the latter. This type,

Teste Scott and Osborn.