QA THE OSTEOLOGY OF ELOTHERIUM. 
few caudals) and part of the hyoid apparatus, the skeleton is complete; it is represented 
in Pl. XVII, which will enable the reader to judge of its unusual state of preservation. 
Additional material, belonging to several species, will also be made use of for purposes of 
of comparison, but the description will deal almost exclusively with the White River 
forms. 
The Artiodactyla may almost be designated as the despair of the morphologist. So 
manifold are the forms which this puzzling group has assumed, and so variously are the 
characteristics of its minor groups combined, that the confusion seems hopeless. ‘The 
only way in which this tangled skein can be unrayeled and its many threads separated 
and made straight, is by the slow but sure method of tracing. the phylogenetic develop- 
ment of each family step by step from its incipient stages. Many years must pass before 
sufficient paleeontological material has been gathered to make this possible, but already 
some progress has been made in the work. Each successive form in a series, as soon as it 
is recovered, should be fully described and illustrated for the benefit of other workers, a 
necessity which must excuse the minuteness of detail into which the following deserip- 
tion enters. For the sake of conyenience the entire bony structure of the animal will be 
described, including those parts which are already well known, in order that the reader 
may be spared the trouble of searching through many scattered papers, written in several 
languages. 
I. Tue Deyririon. 
The teeth of Hlotherium are already familiarly known and require but a brief account 
here. The dental formula is I 3, C 4, P 4, M 3. 
A. Upper Jaw.—The incisors, three in number, increase regularly in size from the 
first to the third, the latter being much the largest of the series; it has a conical or some- 
what trihedral crown and resembles a canine in shape and appearance. In some individ- 
uals the crown of this tooth is worn in a peculiar manner, a deep groove or notch being 
formed on its posterior side, in a place where it cannot haye been made by the attrition 
of any of the lower teeth. The other incisors have spatulate crowns, with blunted tips, 
the attrition of use wearing down the apices as well as the posterior faces of these teeth. 
This description applies more particularly to the larger White River species, such as 
Lf. ingens and E. imperator ; in E. mortoni the upper incisors are of more nearly equal 
size and more conical shape. In all, the median incisors are separated from each other 
by a considerable notch, and the whole series is much more extended antero-posteriorly 
than transversely, the external incisor standing behind the second one. I 3 is separated 
by a short diastema from the canine and at this point the premaxillary border is quite 
deeply notched to receive the lower canine. 
The canine is a very large and powerful tusk, with a swollen, gibbous fang; the 
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