228 MEMOIRS OF THE CARNEGIE MUSEUM. 
centra of the vertebra, and by the disarticulation which is shown in some of the 
parts, were immature. 
Against this hypothesis is the fact that a large proportion of this material 
represents animals which were undoubtedly fully adult, as is shown by the condition 
of the bones. We have in fact material representing both the juvenile and the 
fully adult stage of animals which in many of their proportions show that they 
were not much more than half the size of the adult M. elatus, as represented in 
the articulated skeleton, approximating in size a large tapir, while M. elatus in its 
fully adult form was as large as a rhinoceros. 
Against this hypothesis is the fact that these bones agree in presenting im- 
portant differences in the form of the last dorsal vertebree, the prezygapophyses of 
the dorsals and first lumbar, the slope of the neural spines of the sacrum, the 
development of the limbs, including the scapula, and the contour of the upper 
part of the cranium. In the original description of M. petersoni the writer called 
attention to the fact that the interparietal in the latter species is subquadrate in 
outline, while in M. elatus it is triangular in all stages of development. This 
distinction appears to hold good in the adults and the young of the two forms. In 
our material we have the skull of an adolescent individual of M. elatus (No. 2104) 
in which the temporal ridges come together to form a low sagittal crest as completely 
as in the fully adult specimen (No. 2103) and revealing the interparietal bone as 
having the outline of a narrow isosceles triangle, just as in the latter specimen. 
This specimen shows that the peculiar configuration of the posterior upper surface 
of the skull is as characteristic in young specimens of M. elatus, as it is in old 
specimens. In view, therefore, of the fact, that, in predicating the specific identity 
of the material under discussion with that representing M. elatus, we would have 
to rest simply upon its smaller size, disregarding a number of other obvious differ- 
ences, the writer, after careful consideration, has been led to reject the first hypothe- 
sis as untenable. 
2. Second Hypothesis——The second hypothesis which may be applied to this 
material is, as has been pointed out, that it represents the female sex, while the 
material representing the larger animal represents the male sex. 
In favor of the view that this hypothesis may be correct, it can be urged, first, 
that among mammals the female is generally of smaller size than the male; secondly, 
that the limbs and feet in the female are slenderer and not as heavily built as in 
the male; thirdly, that in the skull there is often a tendency to the development of 
a sagittal crest or other prominences in the male. while such features are absent, 
or less marked, in the female. Allowing full weight to these considerations, it 
