1890.] Geology and Paleontology. 471 
type with Systemodon, through some intermediate forms, as suggested 
by the present writer.? Amynodon is fully elucidated as far as cranial 
structure goes, and its relation to the primitive rhinoceroses pointed 
out. A new Triplopus (T. obliquidens S. & O.) is described, which is 
of larger size than the 7. cubitalis of the Bridger, and the knowledge 
of the character of the genus is increased. Protoreodon S. & O. is of 
especial interest. It is essentially an Oreodont with the fifth crescent 
(metaconule) of the superior molar present as in the Eocene Artio- 
dactyla generally. It is so much like Oreodon in other respects as to 
indicate its place as ancestral to that form, and it adds one more piece 
of evidence to sustain the view that the tetraselenodont forms are the 
descendants of the pentaselenodont Artiodactyla. The Pvrotoreodon 
parvus S. &, O. was an animal about the size of a raccoon. Lepto- 
' tragulus is an equally interesting form, since it supplies the hitherto 
missing ancestor of Poébrotherium. It is tetraselenodont, and differs 
chiefly from the latter and later genus in the freedom of the ulna and 
radius from each other. Thus the genera of Ungulata of this forma- 
tion extend the phylogenetic lines by one period backwards, or they 
fill gaps hypothetically awaiting occupation. 
The chapter on the evolution of the Ungulate foot, by Prof. Osborn, 
handles the subject in a masterly way. The author admits the funda- 
mental propriety of the system proposed by Cope, but takes some ex- 
ception to an application of it in detail in certain directions. The 
exception on which most stress is laid is the fact that in the equine 
line, after the development of diplarthry in the posterior foot, a ten- 
dency to revert to taxeopody appears. Prof. Osborn states that the 
above-mentioned system becomes here ‘‘ not strictly applicable ; ” but 
as he has not demonstrated a return to absolute taxeopody, and as he 
admits the fundamental conformity of the system to nature, the objec- 
tion cannot be said to have much weight. The condition in Equus 
shows only the effect of the increased dimensions of the median digit 
and the corresponding elements of the second tarsal row, to which the 
first tarsal row does not fully correspond. 
Prof. Osborn also objects to Cope’s theory of the origin of diplarthry 
by torsion, stating with reason that were this the only movement, the 
metarso-tarsal articulation cannot be accounted for. In other words, 
were the tarsus and carpus to be rotated externally on the metatarsus 
and metacarpus," dislocation of their articulation would take place. 
Prof. Osborn called my attention verbally to this point, and as I had 
not previously considered it, I supplied what I thought to be an ade- 
2 Article Perissodactyla, AMER. NATURALIST, 1887, December. 
