A new order of extinct Eocene mammals (Mesodactyla) / by O.C. Marsh.

  • Marsh, Othniel Charles, 1831-1899.
Date:
[1892]
Catalogue details

Licence: Public Domain Mark

Credit: A new order of extinct Eocene mammals (Mesodactyla) / by O.C. Marsh. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    Ihe feet are pniiiitive in type, and were apparently diffiti- gra( e, yet tlie tei'inmal ])luilan^?es were not encased in lioofs or in claws, but possessed an intermediate character, whicli has suggested the ordinal name. Their extremities are thin somcMdiat expanded, and more like those of Primates than of any other group. They were apparently covered by thin nails Uie femur has a third trochanter, near the middle of the shaft. The tibia resembles that of a carnivore. The fll)ula is complete, well developed, and at its distal end is expanded, and meets both the astragalus and the calcaneum. The hind foot is represented in figure 2, and presents several points of interest. The astragalus and calcaneum are very similar to those in Rodents. The navicular is supported entirely by the astragalus, and the cuboid by the calcaneum. On the tibial side below the navicular, and supported by it, is a small bone {x) having beneath it the entocuneiform. It meets the middle cuneiform also. This bone has apparently not been kn own hitherto, and may be called the epicuneiforra. It may perhaps, correspond to what is known as the tibial sesamoid in Rodents and Carnivores, but its position and connection in the present tarsus are quite different. Moreover, a distinct tibial sesamoid is also present inside and behind the epicuneiform, and hence not shown in the figure. _ In the feet of Meniscotherium, there are five well-developed digits in raanus and pes. The structure of the carpus and tarsus agrees in general with that of Hyracops^ but in some points, there appear to be differences of importance. The order Mesodaotyla contains at present the two genera Meniscotherium and liyracojps, which include primitive mam- mals of small size, having the full number of forty-four teeth in continuous series, with the premolars and molars similar in type to mammals known as Ungulates. The limbs and feet are primitive in type, unlike those of any known Ungulates, and resemble those of Carnivores, or the early Primates. This order stands in somewhat the same relation to the typical Ungulates that the Tillodontia do to Rodents, and the OhalicothericV^ to Edentates. Yery briefly defined, tlie Meso- dactyla may be considered as having the ungulate type of dentition, with the limbs and feet of early Primates ; the Chalicotheria as combining an ungulate dentition with the feet of Edentates; while the Tillodontia unite with the rodent dentition the extremities of primitive Carnivores. These three orders are quite distinct from each other and from the remain- * The name first proposed for this order, Chalicotheroidea, Gill, 1872, may appropriately be shortened to Chalicolheria. The term Ancylopoda more recently given is preoccupied. The genus Moropus is not identical witii Ghalicotherium as has been stated, and Moroiherium, which has also been referred to the latter genus, is a true Edentate allied to Myhdon.