Part 5

This species is intermediate in size between _H. agrarius_ and _H. modestus_. As in the latter species; the basal ridge extends around the entire crown of the tooth, with a small interruption on the antero-external lobe. It is more pronounced, in front and less behind than in _H. imperialis_. The anterior conical buttress is not nearly so well developed as in _H. imperialis_; the valleys are wide, and the descending fold from the antero-external lobe is obscure, scarcely marked at all in the first. The transverse crests are strongly arched forwards, and the anterior is very much the longer. The peculiar feature of the species is the position of the postero-externa, lobe, which is pushed to the posterior aspect of the tooth; this is carried so far in the last molar that the two external lobes stand on nearly the same transverse line. The antero-external lobe is much the highest point in the tooth, and its the first molar is remarkably sharp. The enamel is considerably wrinkled, but not nearly so much as in _H. modestus_.

_Measurements._

M. Length of first molar ·012 Breadth of first molar ·0155 Length of second molar ·015 Breadth of second molar ·017 Length of third molar ·0145 Breadth of third molar ·017

From Cottonwood Creek.

Hyrachyus nanus, Leidy.

Pr. Ac. Nat. Sc., 1872, 20.

Henry's Fork.

Hyrachyus agrarius, Leidy.

Pr. Ac. Nat. Sc., 1871, 229.

Henry's Fork.

Hyrachyus eximius, Leidy.

Pr. Ac. Nat. Sc., 1871, 229.

Cottonwood Creek.

Hyrachyus crassidens, _sp. nov._

Established on the left and a portion of the right ramus mandibuli, containing the first molar.

The jaw presents some differences from the form common in Hyrachyus. The alveolus is more curved, especially near the ascending portion of the ramus; the jaw is narrower, and is especially contracted near the incisive alveolus. The lower margin of the ramus is but slightly curved. The ramus itself thickens regularly from before backwards, and is thickest at the last molar; beyond which it becomes thinner again. The depth of the jaw is comparatively small, and the teeth have a forward inclination; the mental foramina are not so large as in other species of _Hyrachyus_, and are four in number.

The incisors are small, arranged in a semi-circle, and have very much compressed fangs, with somewhat procumbent crowns; the canines are small, and form a continuous series with the incisors. There is a diastema between the canine and first premolar; but one which is shorter than in known species of the genus, and which does not indicate the usual curve of the upper border of the ramus.

The molar series, (so far as can be judged from their broken condition), are much the same as in other species; except, perhaps, in the greater relative size of the posterior lobes, and in the more complete closing of the median valley by the junction of the two lobes externally. The ridge which curves downwards and inwards from the external point of the anterior lobe is unusually well developed, and curves across the entire crown. The cingulum is feebly shown; it appears on the anterior and posterior ends of the tooth (first molar) and for a very short distance on the outside of the tooth opposite the median valley; there is no trace of it on the inner side.

_Measurements._

M. Length of entire molar series (about) ·080 Length of premolar series ·033 Length of true molar series (about) ·047 Fore-and-aft diameter of fang of canine ·008 Distance between canine alveoli ·009 Length of diastema ·115 Length of the first molar ·015 Breadth of the first molar ·010 Depth of ramus below last molar ·0245 Depth of ramus below last premolar ·0215 Greatest thickness of ramus ·017 Length of symphysis (about) ·030

Found at Henry's Fork.

Hyrachyus -------- ?

There are three premolars of the upper jaw which we cannot refer to any known species; but they are too imperfect to justify the formation of a new species for their reception.

What appears to be a first premolar is inserted by a single broad fang; it is too much worn to show the characteristic enamel folding, and is peculiarly broad and short. The second premolar shows the usual foldings of _Hyrachyus_; save that a distinct valley enters the tooth from the posterior edge, running forward to the transverse crest, and inclosed by the curving backwards of the internal cone. The crown is nearly square; the basal ridge is scarcely marked at all.

_Measurements._

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.