Chapter M

Breadth of ilium at acetabulum ·0065
Vertical diameter of acetabulum ·007
Transverse diameter of acetabulum ·009
Fore-and-aft diameter of shaft of femur above trochlea ·0065
Transverse diameter of shaft above trochlea ·0065
Width of trochlea ·004
Length of trochlea ·0075
Breadth over condyles ·010
Breadth of inner condyles ·005

OLIGOTOMUS, Cope.

Ann. Rep. U. S. Geol. Survey of the Terrs., 1872, p. 607.

"Molars constructed much as in _Hyopsodus_ and _Lophiotherium_, viz., with two external subtrihedral cusps which wear into crescents, the posterior connected by a low oblique ridge with the basis of the anterior cone of the inner side; the latter with two conic cusps. It differs from these genera and from _Orotherium_ in the possession of two premolars; the inferior molars are probably six, leaving four true molars."

Oligotomus cinctus, Cope.

Loc. cit.

Represented in our collection by the penultimate lower molar, and a caudal vertebra, which has a remarkably long, slender, and simple centrum, with rudimentary metapophyses.

OPISTHOTOMUS, Cope.

Wheeler's Survey, Pal. v. iv., pt. 11, p. 152.

"The inferior lower molars do not display a bifid or double anterior cusp; and the crowns exhibit two anterior cones, and an inner cone and outer crescent posteriorly. The posterior crescent is well defined, and is continued on a narrow crest to the anterior inner tubercle. The posterior molar presents the peculiarity of a series of three cusps in one line, the median having another or lateral cusp near it."

This genus has hitherto been found only in the Wahsatch formation; but we have discovered it to be represented in the Bridger series by the species _O. astutus_, Cope. Our specimen consists of a part of the ramus mandibuli containing a single molar tooth.

CARNIVORA

SINOPA, Leidy.

A genus of small carnivorous animals, which Dr. Leidy regards as intermediate between the recent _Canis_ and the extinct _Hyænodon_. Owing to the fragmentary condition of the remains found, no satisfactory generic definition has been given.

From the portion in our collection, we are able to throw some further light upon the genus, summing up the generic characteristics thus: Small carnivores, which have the last upper premolar as sectorial (thus differing from _Hyænodon_), the other premolars simple and conical.

The sectorial is shorter, antero-posteriorly, than the preceding tooth; has a short blade of a single lobe, and a large cusp developed from the posterior part; a cingulum surrounds the entire crown. The _lower_ sectorial has the blade of a single lobe, and with a short heel.

Sinopa rapax, Leidy.

Proceedings of Ac. Nat. Sc., 1871, p. 115.

In addition to the molars of the lower jaw, described by Dr. Leidy we have what corresponds to the third and fourth premolars of the fox, their dental formulas being probably the same.

The third premolar is small and pointed; differing from the corresponding tooth in the fox, (1) in its being less compressed, (2) in its shorter antero-posterior diameter, (3) in the straighter and more nearly equal margins, and in (4) the absence of a posterior heel.

The tooth is inserted by two fangs, as in _Canis_ and _Hyænodon_. The posterior shows a rudiment of a third, which is connate with its entire length above the alveolus. There is an indistinct cingulum around the entire crown.

The fourth premolar has a very curious shape. The blade of this tooth resembles the crown of the third, but is smaller. It is inserted by three fangs, the disposition of which is opposite to that in _Canis_, the internal, being on the same transverse line as the posterior external, instead of the anterior, as in _Canis_. From the internal fang arises a sharp cusp, which is nearly as large as the blade of the tooth, the two are connate at base. The anterior face of the crown is much worn, and there is a small anterior heel formed by the basal ridge. The cingulum is complete all around.

The _maxillary_ does not show the outward bulge at the third premolar, which is so marked in the fox. The alveolus is straighter, and the palatine plates are comparatively thicker and flatter. The infraorbital foramen is oval, and not so much compressed as in the fox, to which it corresponds very nearly in position, though situated slightly forward as in _Hyænodon_.

_Measurements._

_Upper Jaw._ M.

Length of third premolar ·007
Breadth of third premolar ·004
Length of fourth premolar ·007
Breadth of fourth premolar ·007

_Lower Molars_, from Dr. Leidy.

Length of last premolar ·0075
Length of first molar ·009

These exhibit nearly the same proportionate size as in the gray fox.

_Genus_ ----. _Species_ ----.

_Sacrum_ (Plate IX., Fig. 8).--This peculiar sacrum is composed of only one true vertebra; there may have been one or more pseudo-sacrals, but this is not certain.

The centrum is very long, strongly depressed, and straight on the inner margin, not curved as in the sacrum of most mammals. The anterior articular face is much depressed, and is one third larger than the posterior. The neural canal is low and subtriangular, resembling very much that of _Canis_. The pleuropophysial plates for articulation with the ilia are large and stout. The laminæ are heavy and concave on their upper side, supporting a very long, stout spine, which is retroverted and decidedly tuberous at the end.

The pedicles are deeply notched behind; and on the fore part, just inside the metapophyses, there is a deep fossa.

The chief features of this sacrum are decidedly carnivorous; but to what genus or family it should be referred we are unable to say.

It has some of the characteristics of _Canis_, but the length and retroversion of the spine, as well as the size of the centrum, prevent this classification. In the general form of the pleuropophysial plates it approximates to the _seals_; while in its angle and curvature, it partakes of the character of the _Ursidæ_.

The chief point of interest in this fossil centres in the fact that it was found only a few feet from the brain cast that is described below.

_Measurements of Sacrum._