Chapter M

Third metatarsal, total length ·090
Third metatarsal, transverse diameter, proximal face ·017
Third metatarsal, transverse diameter, distal end ·019
Second phalanx, length ·024
Third phalanx ·014
Total estimated length of manus (6 inches) ·155

Palæosyops vallidens, Cope.

Proceedings Am. Phil. Society, 1872, p. 487; Hayden's Survey, 1872,
p. 572.

_Specific characters._--Founded on details of dental structure. Superior size. In the upper molars two strong transverse ridges connect the inner tubercle with the outer crescents, inclosing a pit between them. In the premolars the outer crescents fuse almost into a single ridge. These united crescents are relatively narrower. The inner molars are also narrower, and the posterior tubercle of the last is an elevated cone.

Fragment of a lower jaw containing the two posterior molars. The measurements of the teeth are identical with those given by Professor Cope. The depth of the ramus below the last molar is three and one third inches, and it is slightly heavier below than in _P. major_.

_Restoration of Palæosyops._--The elevation of _Palæosyops major_, which is taken as a type, was approximately the same as that of _Palæotherium magnum_ as restored by Cuvier.

The nearly complete ilium, femur, tibia, and tarsus of _P. major_ in our possession, enable us to make the following estimate of the elevation of the hind quarters.

INCHES.

_Pelvis_, height of ilium above acetabulum 9
_Femur_ and _tibia_ by actual measurement, allowing for
bend at the knee 26
_Tarsus_, from astragalus, and cuboid 3
_Metatarsus_ and _phalanges_ (estimated from manus of
_P. paludosus_) 8
--
46

It is probable that the fore-shoulders were of the same height. From the atlas, axis, and from other cervicals, several dorso-lumbar vertebræ, and part of the sacrum, a rude estimate places the length of the animal at sixty-seven inches, or nearly six feet, including the head, and excluding the tail.

In general features it strongly resembled the tapir, with stout body, slender tail, and very short neck, compensated by a proboscis of considerable length. In comparing the heads of the ancient and modern representatives of this class of Perissodactyles, the points of contrast are the wide stout zygomas, the deep temporal fossæ, the protruding nasals, and the narrow gaps in the dental series of the older type.

There are also strong points of resemblance in the structure of the _Palæosyops_ and _Palæothere_. Some of the cranial homologues have been detailed by Dr. Leidy. The femur of the Palæothere is shorter, the tibia longer; they are both much stouter. The tarsus of the _Palæosyops_ is narrower, while the remainder of the Pes is very similar. The pelvis, and particularly the ilium and acetabulum of _Palæosyops_, is more palæotheroid than tapiroid. The similarity of the fore-shoulder in the two types is also striking. The neck of the _Palæosyops_ was probably shorter.

In the descending series of _Palæotheres_ terminating in the diminutive _P. minus_, and characteristic of the upper eocene of France, we have an interesting counterpart in the large family of _Palæosyops_, of equal diversity of size and characteristic of our lower eocene, and it is hoped that future discoveries will render a complete discussion of this interesting coincidence possible.

LIMNOHYUS, Leidy.

Proceedings Academy Natural Sciences, Phil., 1872, p. 242;
_Palæosyops_; Marsh, American Journal Sci. and Arts, 1872, p.
122; Hayden's Survey, 1872, p. 592, Cope.

_Generic characters._--Resembles _Palæosyops_ in general features. Distinction founded on the possession of two conic tubercles, of the inner series, on the last superior molar instead of one.

Limnohyus laticeps, Marsh.

_Palæosyops Laticeps._ Am. Journ. of Sc. v. iv. p. 122.

_Specific characters._--"Cranium is broad. The zygomatic arches much expanded. The nasals are narrow and elongated, and more like the corresponding bones in Hyrax than those in the larger pachyderms."

Fragments of the upper molar and premolar series. The species is determined by measurements given by Dr. Leidy of the second upper molar.

LEUROCEPHALUS, _Gen. Nov._

Established on a specimen having a nearly complete dentition, and portions of the cranium.

Dental Formula: (I. 3/3, C. 1/1, Pm. 4/4, M. 3/3,) × 2 = 44.

Upper incisors acute, with strong posterior ridges, lower incisors compressed and laniariform, canines compressed, with serrated cutting edges; first upper premolar with rudimentary anterior lobe, last upper molar with rudimentary postero-internal cusp. Molars constructed as in _Palæosyops_, but higher, with sharper cones and more erect external lobes. Internal median valley very much deeper. Little or no depression at the forehead; zygomatic arch round, comparatively straight and does not project outwards, and with obscure postorbital process. Premaxillaries short and straight. Mandible with nearly straight lower margin, and shallow masseteric fossa; mental foramen single.

Leurocephalus cultridens, _Sp. Nov._

This species was considerably larger than _Palæosyops major_, with which it shows affinities, as also with _Titanotherium Proutii_. See Plate IV.

The _Frontal_ is narrow and flat, or slightly arched. It sends out a strong curved postorbital process, which projects outwards, but not so much outwards as in _Palæosyops_. The sagittal crest runs back from the postorbital in a low rounded ridge, rising but little as it recedes. The superciliary ridge is distinct, and is pierced by two small venous foramina. The shape of the entire bone is much as we find it in the dog, except that it does not arch so much. One of the most marked differences between this genus and _Palæosyops_ is here shown. In the latter the forehead rises abruptly from behind the orbit, while in the former there is only a slight rise for some distance behind the orbit. As the bone is broken here, it is impossible to state whether there is any rise at all. Probably not, however. Below the postorbital process, on the lateral aspect of the bone, there is a low ridge which separates the orbit from the temporal fossa. The under surface of the frontal is smooth.

The _Nasal_, of which but a small portion is preserved, is flat on top, but bends downward at the angle.

The _Maxillary_ is long and stout, over the last and penultimate molars it broadens to form the floor of the orbit, which is of unusual size. Between the canine and the malar the maxillary arches inwards, forming a perceptible concavity, at this point it reaches its greatest vertical height, as it rises to join the nasal. The alveolar border is curved in two directions, one with the convexity outwards, and the other downwards. It exhibits no emargination in the diastema between the canines and molars. The palatine plate is long, thick, and narrow; it is flat fore-and-aft, but concave transversely, owing to the elevation of the alveolus; the suture also is raised slightly. In thickness it varies; being thickest between the canine and the second premolar, and beyond this becoming thinner. Its forward termination seems to be obtuse, running for a short distance along the premaxillaries. The infraorbital foramen is large, situated over the fourth premolar, lower down, and nearer to the malar than in _Palæosyops_.

The _Premaxillary_ is rather short and stout; it is slightly compressed, and ends in a sharp keel on top. Although not so thick as in _Palæosyops_, it is much larger vertically. The inner surface is ridged; it has no palatine process, and no spine, so that the incisive foramen is large and undivided. There was no symphysis between the two premaxillaries, they do not show even any articular faces for each other, so that they may not have been in contact during life. The animal was adult, but not old. The incisive alveolus is short and straight, and is but little out of the line of the molars. There is a very long diastema between the incisors and the canine, and here the premaxillary is deeply notched to allow the passage of the lower canine.

The _Malar_ is very different in shape and size from the same bone in _Palæosyops_. Its facial extent is greater, as it articulates with the maxillary as far forward as the first molar. It is not so broad from above downwards, but is considerably thicker; the articulation with the squamosal is by flat surfaces. The postorbital process is small and indistinct. The orbit is of great size; its breadth, shown by the maxillary floor is unusual, but its fore-and-aft diameter is extreme, being nearly twice as great as in a large _Palæosyops major_.

The _Squamosal_ is a short, slender, and trihedral bone; it is not at all like the squamosal of _Palæosyops_. It is smaller even than that of _P. paludosus_; its outer margin is not turned up, making the upper surface concave; it does not project much outward, and scarcely at all downwards, as this is rendered unnecessary by the flatness of the forehead. The chief difference, in the size of this bone, between this genus and _Palæosyops_ is found in the much greater vertical and lateral diameter of the latter; in length they are about equal. The glenoid cavity is large, shallow, and transverse.

As a whole, the zygomatic arch is comparatively slender; it is rounded, and does not project much laterally from the side of the head; but its chief peculiarity consists in the fact that it is nearly horizontal. It is thus altogether different from the arch of _Palæosyops_.

The _Occipital Condyles_ are different from those of that genus. They are flatter, shorter from above downwards, and proportionately much smaller; but are expanded laterally in much the same manner. At the border of the foramen magnum they are gently rounded instead of having a sharp angle. These borders are segments of circles, but as the condyles are broken from the rest of the occiput, we cannot infer from this the shape of the foramen.

The _Mandible_ is long, stout, but comparatively shallow. It is of nearly uniform thickness throughout, and does not exhibit the thickening of the alveolus and thinning, of the lower margin, which is shown in _Palæosyops_, especially in _P. major_; and the downward and inward arching of the alveolar border is not marked to the same extent as in that species. The lower border is nearly straight, and has but one slight emargination just behind the symphysis. The ascending portion of the ramus rises near the last molar quite abruptly, and makes an angle of nearly 90° with the horizontal portion. The masseteric fossa is shallow and obscure, and the anterior bounding ridge is almost obsolete. This part of the jaw is very different from any of the species of _Palæosyops_. The symphysis is quite long, and extends back as far as the second premolar; the chin is probably rounded. The dental foramen is situated high up in the ascending portion of the ramus; the mental foramen is placed below the second premolar; it is large and single.

_Cranial Measurements._