_Specific characteristics._
The _premaxillaries_ (See Plate X., Figs. 1-4) are long, with a wide,
strongly upward curving process; from the premaxillary symphysis to
the third incisor, they increase in thickness; then narrow slightly
to the maxillary symphysis. The upper margin of the process is smooth
and narrow, curving very gradually upwards; while the lower margin
runs more abruptly up, giving to it a strongly pointed curvature.
The outer surfaces are smooth and slightly convex; the inner are
marked by a wide deep canal running from just behind the premaxillary
symphysis into the maxilla.
The great size of this groove may perhaps be a generic
characteristic; as it is much larger than in _Protolabis_ (Plate X.,
Fig. 6), the modern camels, or any of the other ungulates, recent or
fossil, that have come under our notice.
The premaxillaries do not co-ossify, as the spines are smooth on
their inner side. Just outside the spines, and in front of the
deep groove, the bones are marked by numerous small pits for the
attachment of the muscles of the upper lip.
The portion of the maxilla in which the first premolar is situated,
shows that the maxillaries swell considerably on their alveolar
border; for the canine then narrow gradually for the first premolar
diastema; then swell for its reception, and become very thin in the
diastema between the first and second premolars.
_The Teeth._--I. 3/?, C. 1/?, Pm. (1+)/?, M. ?/?.
The incisors have quite deep fangs, are cylindrical in shape,
vertical in position, and have no basal ridge.
The crown of the first is worn perfectly smooth, flat across the
top, unlike the wearing exhibited in any modern form that has come
under our notice. The others have their crowns broken, but indicate a
nearly subequal series, with the last slightly the largest.
The canines have long, recurved compressed fangs, with rather short
crowns, which are circular in section. They are much larger than in
_Procamelus_ or _Protolabis_ of Pliocene, or the modern camels. (See
X., Figs. 3, 5, 6, 7.)
The first premolar is the only one of the molar series preserved; its
crown is chipped considerably, but nevertheless shows that it had one
fang and no basal ridge. It resembles the incisors in form, but is
slightly narrower and longer.
The genus _Ithygrammodon_ has been placed for the present under
the _genera incertæ sedis_; but its chief features point to an
unmistakable affinity with the ruminants. It is probable that
_Ithygrammodon_ was the representative and the ancestor, in the
eocene age, of that type of ungulates of which the camel and llama
are the modern forms.
The line of descent of the camels was first indicated in the
Proceedings of the Philadelphia Academy, 1875, p. 262; then in the
Bulletin No. I, 1874, p. 25, of the U.S. Geological Surveys of
the Territories, (Government Report of Explorations of 1873, pp.
498-500); and lastly in U. S. Geographical Surveys of Territories,
vol. iv., pp. 341-44.
In the above writings, Professor Cope traces the development of the
modern camels from the miocene genus _Poëbrotherium_, Leidy, showing
the modifications found in _Procamelus_, Leidy, and _Protolabis_,
Cope, of the succeeding age.
From the last work (Wheeler's Survey, vol. iv., p. 342), we will
quote at length:
"The evolution of the existing types of Camelidæ is a good
illustration of the operation of the laws of acceleration and
retardation. In evidence of this we may follow the growth of the foot
and dentition of the most specialized, and therefore the terminal
genus of the series, the American _Auchenia_ ----. It is well known
since the time of Goodsir, that the embryos of ruminants exhibit
a series of superior incisor teeth which disappear early. It is
probable, but not certain, that in the miocene genus _Poëbrotherium_,
as in various contemporary selenodont artiodactyla, the superior
incisors persisted. I have, however, discovered that these teeth
persisted in the Loup-Fork genus _Protolabis_ during adult life.
I have found that one (the second) of these teeth in _Procamelus
occidentalis_ persisted, without being protruded from the alveolus,
until nearly adult age.
"In genera (_e.g._, the bunodont artiodactyla) where the incisors
are normally developed, they appear at about the same time with the
other teeth, and continue to develop to functional completeness.
This development is retarded in _Protolabis_, since they are not so
matured as to remain fixed throughout life in their alveoli.
"In _Procamelus_, the retardation is still greater, since the first
incisor reaches very small dimensions, and is, with its alveolus,
early removed; while the second incisor only grows large enough,
and for a sufficient time, to occupy a shallow alveolus, without
extending beyond it. In the first incisor the process of retardation
has reached its necessary termination, _i.e._, atrophy or extinction;
while in the existing _Camelidæ_ the second incisor also has
disappeared the same way. In ruminants other than _Camelidæ_, the
third or external incisor has undergone the same process, while in
the _Bovidæ_ the canines also have been retarded in development, down
to atrophy."
Professor Cope continuing the discussion of the teeth of these types,
generalizes: "From these and many analogous cases, the general
law may be deduced that _identical modifications of structure,
constituting evolution of types, have supervenea on distinct lines of
descent?_"
This summary of Professor Cope's conclusions has been necessary to
show clearly in this series the probable place of the eocene genus.
From the shape of the premaxillaries and maxillaries, and the
form and position of the teeth the camelline affinities have been
indicated. Future research must be relied on to throw light upon the
structure of the limbs and the axial skeleton; as it is from these
that the conclusive proof must be deduced that _Ithygrammodon_ is the
ancestor of the camels, and that _Poëbrotherium_ is the second link
in the chain, instead of the progenitor.
_Measurements of Ithygrammodon cameloides with Protolabis._
--------------------------------------------+----------------+-----------
|Ithygrammodon c.|Protolabis.
+----------------+-----------
Length of alveolar border of premaxillaries | ·035 | ·049
Greatest height of premaxillaries | ·017 | ·022
Greatest width of premaxillaries at last | |
incisor | ·013 | ·01155
Thickness of premaxillary process | ·010 | ·017
Length of diastema between first and second | |
incisors | ·0035 |
Length of diastema between second and third | |
incisors | ·003 |
Length of diastema between third incisors | |
and canine | ·004 |
| |
_Teeth._ | |
| |
Fore-and-aft diameter of first incisor | ·007 |
Fore-and-aft diameter of second incisor | ·0075 |
Fore-and-aft diameter of third incisor | ·008 | ·011
Fore-and-aft diameter of canine | ·012 | ·012
Fore-and-aft diameter of first premolar | ·011 | ·011
Transverse diameter of first incisor | ·007 |
Transverse diameter of second incisor | ·0073 |
Transverse diameter of third incisor | ·0085 |
Transverse diameter of canine | ·0113 | ·009
Transverse diameter of first premolar | ·005 | ·007
--------------------------------------------+----------------+-----------