1392.] . Geology and Paleontology. 943 
animal becomes a Protohippus. He had not observed this to take 
place in any other species referred to Hippotherium. In both these: 
stages the enamel borders of the lakes are more or less plicate, and 
the posterior loop of the anterior lake is present. With further wear 
the plications, including the loop, disappear, when the molars agree in 
their characters with the Protohippus parvulus Marsh. These obser- 
vations were based on specimens from the Loup Fork beds of 
Nebraska, Kansas, Colorado and Texas, where the species is abundant. 
The speaker exhibited the molar dentitions of three colts from 
Wyoming and Texas, and a part of one from Colorado, all from the 
Loup Fork beds. He showed that these represent the genera Mery- 
chippus, Parahippus, Hypohippus, and Anchippus of Leidy, and six 
species of the same author. He thought it probable that Anchippus 
‘belongs to a colt of Hippotherium, and Parahippusand Hypohippus to 
Protohippus, while he was not certain as to the reference of the type of 
Merychippus (M. insignis). He pointed out that the characters of 
the individual temporary molars differ in the different teeth of the 
series, and also differ at different stages of wear. As with the perma- 
pent dentition, in some species the temporary molars are always 
simple, while in others the enamel borders are more complex. In the 
latter case the pattern becomes more simple in some respects with pro- 
longed wear. He was able to correlate the temporary and permanent 
dentitions of Protohippus perditus Leidy with certainty, and those of 
P. pachyops Cope and P. mirabilis Leidy with much probability. 
Prof. Cope further pointed out that the temporary dentition in these 
- three-toed horses is more simple than that of the adult, in some cases 
resembling very closely the permanent dentition of the ancestral 
Anchitherium in molar structure. In this the horses differ from the 
higher Artiodactyla, where the wi ed molars are equally complex 
or more so than the permanent molars. 
The accompanying plates (XXV, XXVI) illustrate the statements 
-made above. In Plate XXV we have the gradations in the pattern 
of the grinding surface of the molars in the Protohippus placidus 
Leidy. Figs. 1 and 2 represent the more complex hippotheroid stage 
of early wear, and in Fig. 3 a simpler stage of the same. Figs. 4, 5, 
and 6 represent the more worn protohippoid stages with greater and 
less complicity of pattern. That individuals differ as to the stage at 
which this occurs is shown by Fig. 6, where the crown is less worn 
than in Figs. 4 and 5. In Fig. 7 we have an old animal with crowns 
fully worn, showing the full protohippoid pattern, with simple pattern. 
Fig. 8 is the corresponding inferior series. All natural size. 
