Chapter M

Length of first premolar ·011
Breadth of first premolar ·016
Length of second premolar ·012
Breadth of second premolar ·014

HELALETES, Marsh.

Am. Journ. Sc. and Arts, 1872, p. 218.

Additional material enables us to give further characteristics of this genus, which may be thus defined: Mammals, allied to _Lophiodon_, having a third lobe to the last lower molar, short symphysis, and with the teeth of the lower jaw forming a continuous series. Astragalus has narrow, oblique condyles.

? - ? 1 - 1 4 - 4 3 - 3
Dental formula: I. -----, C. -----, Pm. -----, M. -----.
3 - 3 1 - 1 4 - 4 3 - 3

Synopsis of Species.

A. Teeth small; a small tubercle on the outer margin,
between the cusps of last upper molar _H. boöps_.

B. Teeth much larger and proportionately broader; no
tubercle on last upper molar _H. latidens_.

Helaletes latidens, _sp. nov._

_Lower Jaw._--The ramus is stout, and resembles in general form that of _Hyrachyus_, but the alveolar border is straighter than in that genus; the ascending portion forms a right angle with the horizontal portion, and the masseteric fossa is deep. The symphysis is comparatively short, and there is complete bony union between the rami, which do not diverge so much as in _Hyrachyus_. As in that genus, there are several small mental foramina beneath the premolar series. The dental series forms a nearly continuous line, the only diastema being a very short one between the first and second premolars. The _incisors_ have compressed fangs, and are arranged in a slight curve. The canine is small and subcircular at base. The _premolars_ also are small; they somewhat resemble those of _Hyrachyus_ in shape, but are smaller, and have the posterior lobe not so well developed. The first is implanted by a single fang, the others by two. The _molars_ (except the last) consist of two pyramidal lobes, which form transverse cutting-crests, as in _Tapirus_ and _Hyrachyus_. The basal ridge is shown on the anterior and internal sides only. Externally the lobes meet at their bases, inclosing the valley on that side. The last molar is by far the longest of the series, and consists of three lobes, of which the anterior is the largest, and the posterior the smallest. The basal ridge at the anterior part of the tooth is very strongly marked.

_Upper Jaw._--The molars increase in size from first to last with considerable regularity. The premolars are small, and, as in _Hyrachyus_, have one single internal cone, which is joined by transverse ridges from the two outer cusps; the anterior accessory tubercle is well developed. The first molar is considerably larger than the last premolar; it is much worn, and does not show anything more than that there are two internal cones. The anterior conical buttress is very largely developed throughout the true molar series. The second and third molars resemble almost precisely the corresponding teeth of _Hyrachyus_. Their shape, however, is more nearly square than is usual in that genus.

_Measurements._

UPPER JAW.
M.
Length of true molar series ·036
Length of last molar ·0135
Width of last molar ·015
Length of second molar ·012
Breadth of second molar ·015
Length of first molar ·0105
Breadth of first molar ·012
Length of last premolar ·009
Breadth of last premolar ·011
Length of third premolar ·008
Breadth of third premolar ·009

LOWER JAW.
Length of true molar series ·042
Length of last molar ·018
Breadth of last molar ·009
Length of second molar ·013
Breadth of second molar ·008
Length of first molar ·012
Breadth of first molar ·008
Length of third premolar ·008
Breadth of third premolar ·005
Diameter of canine at base ·006
Length of symphysis (about) ·027

Found at Henry's Fork.

ARTIODACTYLA.

GENERA INCERTÆ SEDIS.

ITHYGRAMMODON, _gen. nov._

Our fragments of this fossil show an animal about the size of the llama, and approximating more closely to the _Tylopoda_ than to any living forms.

The genus is established upon the two premaxillary bones, containing the incisors, parts of the maxillaries, the canine and the first premolar; besides fragmentary portions of the palatine plates. As the peculiar features of these remains render them unique, we are warranted in establishing the genus _Ithygrammodon_.

_Generic characteristics._

The premaxillaries are long, narrow, and very straight, bent slightly in on their own axis as in _Camelidæ_, but expanded laterally at the end of symphysis as in _Pecora_. The upward processes are sharply defined, with a wide, rounded upper border.

The upper incisors are six in number, and _are placed nearly in a straight line fore-and-aft_, separated from each other and from the canine by small and subequal diastemas.

The _incisors_ are nearly equal in size, cylindrical in shape, are placed vertically in their alveoli, and are slightly recurved.

The spine of the premaxillaries is long and thin, giving very long anterior palatine foramina, as in _Ruminantia_. The premaxillaries, in position, are close together, showing a very narrow muzzle.

The maxillaries have two well-developed canines, with long, laterally compressed fangs, recurved and becoming circular in section at the crowns. The diastema between the canine and the first premolar is proportionate to that in _Procamelus_.

Ithygrammodon cameloides, _sp. nov._

_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
--------------------------------------------+----------------+-----------

AMBLYPODA.

(Cope. Wheeler's Survey, vol. iv., pp. 178 _et ss_.)

"Mammalia, with small cerebral hemispheres which leave the olfactory lobes and cerebellum exposed. The feet short and plantigrade, with numerous (in the known genera, five) digits, terminating in flat, hoof-bearing, ungual phalanges. The seven bones of the carpus distinct, the unciform articulating with both lunar and cuneiform. The astragalus flat, without trochlear surface, and attached to the tibia with very little freedom of movement; its distal extremity divided into two facets, one for the navicular, and the other more or less for the cuboid. Molars inserted with enamel, with wide crowns and transverse crests. A post-glenoid process."

This order falls naturally into two sub-orders:

"I. A third trochanter on the femur, and a fossa for the
round ligament: no alisphenoid canal; superior incisors
present _Pantodonta._

"II. No third trochanter, nor fossa for the round ligament;
an alisphenoid canal; no superior incisors. _Dinocerata._"

This sub-order, Dinocerata, includes at present three distinct genera, _Uintatherium_, Leidy, _Dinoceras_, Marsh, and _Loxolophodon_, Cope (also probably _Megacerops_, Leidy). These three genera, in addition to the characters above given, are marked by the possession of two or more osseous projections from the upper surface of the head; and of these the posterior pair, developed from the parietals, are the largest.

Synopsis of genera of

DINOCERATA.

A. Cervical vertebra? long; median horn-like processes anterior
to the orbit; nasal tuberosities do not overhang
the nasal tips.

(_a_) Last molar, with or without a tubercle, occupying
the entrance of the valley between the lobes,
and but one on posterior basal ridge. Temporal
fossæ not continued beyond the base of the
parietal processes. Occipital condyles projecting.
_Uintatherium._

(_b_) Last molar never has a tubercle at entrance of
valley, and has two on posterior basal ridge.
Temporal fossæ continued very far back; condyles
sessile. _Dinoceras._

B. Cervical vertebræ short; median horn-like processes
directly over the orbit; nasal tuberosities overhang the
entire tips. _Loxolophodon._

Synopsis of species of

UINTATHERIUM.

A. With a tubercle occupying the entrance of the valley
between the lobes of the last upper molar. _U. robustum._

B. No tubercle at the entrance of the valley.

(_a_) Nasals divided by a deep groove; slender zygomatic
arch; dorsal vertebræ compressed. _U. Leidianum._

(_b_) No nasal groove; stout zygomatic arch; dorsal
vertebræ sub-cylindrical _U. princeps._

UINTATHERIUM.

Leidy, Proc. Ac. Nat. Sc., 1872, p. 169.--Cont. to Ext. Vert. Faun.
of Western Territories, p. 93.--Cope, Hayden's Survey, 1872, p.
580, etc.

Skull broader proportionally than in the other genera of the order, ridged and possessing several concavities on the upper surface; zygomas slender and but little curved; temporal fossæ comparatively short and have well-defined superciliary margins. The cervical vertebræ are rather long; the sacrum has four vertebræ; and the tail is quite long, very flat and broad. The tibia has its proximal face divided by a prominent ridge into two parts. The dental formula for the upper jaw is: I. 0, C. 1, M. 6. The molars are small, and increase from first to sixth. The last is much the largest; in it the anterior lobe considerably exceeds the posterior in size, and there may or may not be a tubercle at the entrance of the valley between the lobes; but there is always one developed from the cingulum at the posterior part of the tooth.

Uintatherium leidianum, _sp. nov._[3]

[Footnote 3: This species is respectfully dedicated to Dr. Joseph Leidy, of Philadelphia. The specimens upon which it is established were found on Dry Creek plateau.]

Established upon a head and nearly perfect skeleton of one individual, and parts of two more.

In this species, as in most others of the sub-order, the nasals are of immense length and thickness; they overhang the anterior nares, and project considerably beyond the premaxillaries; they form more than half of the entire length of the skull, articulating with the frontals somewhat behind the orbits; the median suture is distinct throughout. Instead of having an expanding shovel-shaped forward projection, as in _Loxolophodon_, they narrow from the median osseous projections (horn cores?) anteriorly. Above the muzzle they are strongly curved from side to side, on their upper surface forming a continuous arch; on the under surface of each bone is a deep concavity, separated from its fellow by the sutural ridge, which disappears forwards, near the extremity. On the upper surface of each nasal, near the forward end, is a large osseous tuberosity which is directed forwards and outwards; these processes are much smaller than the corresponding ones in the _Loxolophodon_, while they are larger, of different shape and direction from those in _Dinoceras_. In this species they are divided throughout by a deep median groove, which anteriorly becomes a fissure, and separates the extremities of the nasals completely. In front of these tuberosities, the nasals taper very rapidly, and end in sharp points which project downwards and forwards; this portion of the bones is shorter, sharper, and projects more decidedly downwards than in _Dinoceras_. Above and slightly behind the sockets of the cranium, the nasals and maxillaries give rise to the median pair of osseous projections. These are very large, subtrihedral at the base, and project upwards, outwards, and strongly forwards. They differ from the corresponding processes in _Dinoceras_, in their forward projection, in being longer and more everted, and in their approach to each other at the base. They taper quite regularly from base to tip, but do not come to a point; a section here would be subcircular. They are not so long and are not knobbed at the ends as in _Loxolophodon_. Their inner curve is convex, the outer concave; converging below at an angle of 90°, they are united by a strong ridge, which is raised decidedly above the surface of the nasals. At their posterior insertion they touch the frontals, lachrymals, and malars. It is probable that the nasals send up processes on the internal and posterior side of these median projections; the sutures between these and the maxillary portions are marked by slight ridges.

_Frontals._--In our specimen it is very difficult to determine the exact relations of these bones. The nasals narrow at their posterior end, and articulate with the frontals by a V-shaped suture which thus encloses them on each side. It is probable that the frontals run to some point between the posterior or parietal projections. If we have discovered the suture, they overlap the parietals, sending up V-shaped processes, which join the sides of the posterior cranial projections, and form a deep concavity with them. They constitute a large part of the interior wall of the orbit, but have no postorbital processes; the superciliary ridge is very strong, and gradually rises into a large rounded crest, which joins the posterior projection. The frontal eminences are large, and situated immediately over the lachrymals. The upper surface of the frontals is marked by two high ridges, which probably represent the divided parts of the sagittal crest; they rise from the anterior margin of the bones, and are strongest above the frontal eminences. They converge, and then diverging, together forming an X-like curve (without crossing, however), gradually disappear posteriorly. These various ridges form four deep concavities on the upper surface of the skull: (1) The largest already mentioned, between the parietals and the posterior part of the frontals; (2) that marked by the suture between the nasals and frontal, not so large; (3 and 4) two much smaller lateral ridges between the sagittal and superciliary ridges. This great irregularity of the upper surface of the head is, so far as is yet known, peculiar to _Uintatherium_.

_Parietals._--The peculiar feature of these bones is the huge pair of projections (the so-called "posterior horn cores") to which they give rise. The outer margin of these processes is nearly straight; the inner margin for its upper two thirds is also straight, the lower third curving in a high strong ridge to meet its fellow. The anterior face is rounded and produced into the long crest of the frontals, while the posterior face is flattened and produced into a similar but shorter and higher crest which joins the supra-occipital. At the base of these processes their greatest diameter is fore-and-aft, while at the top it is transverse. They project upwards and outwards, but are not curved as in _Dinoceras_, nor are their upper borders so much arched. Their greatest diameter at top is at right angles to the corresponding measurement in this last-named genus. Behind the ridge which connects these projections, the parietals curve sharply upward to a high occipital crest; between which and the projections they form a deep basin whose floor is raised above that formed by the frontals immediately anterior to it. This arrangement differs from that of both _Loxolophodon_ and _Dinoceras_, especially from the latter. The temporal fossæ are of great length, deeply concave, but rather low from above downwards, formed almost entirely by the parietals, and bounded posteriorly by a sharp outward curve of these bones. They are of about the same proportionate length, but higher and very much deeper than in _Loxolophodon_; and are not nearly so long as in _Dinoceras_, in which genus the parietals expand far behind the horn-like processes. In our specimen the posterior part of the fossa is pierced by numerous small venous foramina, and corresponds precisely in every way, save that of size, to the specimen figured by Dr. Leidy. (Cont. to Extinct Vert. Fauna, Plate XXVI., Fig. 1.)

_The Squamosals_ are large and heavy but encroach little upon the temporal fossæ; they are situated directly below the large parietal projections. The glenoid cavity is transverse and straight in this direction, broad and shallow, with no internal process; and the post-glenoid process is long and massive. The zygomatic process is short, stout, high, and trihedral, with a strongly arched upper margin. The articulation with the malar is by a straight, flat face, and anchylosis of the two never takes place. The anterior termination is pointed, and the outer margin is rounded.

_The Malars_ form none of the face. They are long, slender, curved downwards and backwards, and but little outwards. They are longer, straighter, and less curved outwards than in either _Loxolophodon_ or _Dinoceras_; they do not present the sharp angle in the lower margin shown in the latter genus, nor are they so extensively overlapped by the zygomatic processes of the squamosal. Posterior to the molar series, the malars are greatly compressed and very slender, but at the junction with the maxillaries they become much wider and thicker. There is no trace of a postorbital process; and the projections from the under surface of the bone at their junction with the squamosals, so prominent in Dinoceras, are here rudimentary or absent. As a whole, the zygomatic arch is very long, slender, simple, curved upwards and very slightly outwards, so slightly that it is completely overhung by the superciliary ridge and frontal crest.

_The Lachrymals_ are unusually large, and form the anterior part of the orbit; they encroach considerably upon the face, and articulate with the superciliary ridges above. The lachrymal foramen is very large.

_The Maxillaries_ are of great length, being nearly as long as the nasals. They extend somewhat beyond the last molar; but the suture between them and the pterygoids is very obscure. There is a very long diastema between the canine and molar series, and the lower margin is arched upwards; above this, between the orbit and the socket of the canine, there is a large, deep fossa, but we can discover no infraorbital foramina. The sockets of the canines are very large and prominent, they curve upwards and backwards to the base of the median cranial projections, but these are not excavated to receive them, as is the case in _Dinoceras_. The most peculiar feature of the maxillary bones is the pair of large horn-like projections, to which, in conjunction with the nasals, they give rise. These have already been described, and it only remains to add that they correspond in position to those of _Dinoceras_, and are therefore much further forward than those of _Loxolophodon_.

The palatine plates of the maxillaries are long and very narrow; they are concave transversely, and are separated from each other by a high median ridge. The posterior palatine foramina are small.

The _Palatines_ are very short, narrow, and concave, and are separated by a ridge. They are considerably excavated on the posterior border, in this respect differing from _Loxolophodon_.

The _Pterygoids_ and the pterygoid plates of the alisphenoid are compressed; the former join the alveolar borders of the maxillary, which are produced somewhat beyond the last molars. The alisphenoid canal is very large, but rather short.

The _Premaxillaries_ are of very peculiar shape, somewhat like a _u_, with one side--the lower--the longer. The upper portion articulates with the nasals, running along the narial opening to about three inches from the angle; the free portion is short, slender, and tapering; it is curved downwards and slightly inwards; the premaxillaries do not meet, leaving the incisive foramen unenclosed, and are edentulous. At the end of the upper portion there are prominent processes for the attachment of the muscles of the proboscis, which probably resembled that of the tapir.

The anterior narial opening is very large, but is considerably concealed by the overhanging of the nasals. There is no osseous septum between the nostrils. The posterior nares are much smaller, being especially contracted in width; it is divided above by the _Vomer_, which is very short, and does not reach the palatines. The nasal cavity thus formed is long, straight, and gradually narrows backwards.

Of the Mandible we have but a small portion of the right ramus, comprising the part opposed to the upper canine, from the dental canal downwards. The jaw at this place has a large downward-projecting process, very similar to that figured by Marsh in his plate of _Dinoceras laticeps_. This process curves slightly outward, and has its external side convex in both directions, and its internal side convex fore-and-aft, concave from above downwards. The posterior mental foramen, which is the only one preserved in our specimen, corresponds in size and position with _Dinoceras laticeps_. The only difference between the two is a very slight one: in _D._ the curve of the posterior margin of the process is convex; in _Uintatherium_ it is concave; the angle which the process makes with the jaw is also greater.

_Cranial Measurements._