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The zoology of the voyage of H.M.S. Beagle [vol. 1 of 5] cover

The zoology of the voyage of H.M.S. Beagle [vol. 1 of 5]

Chapter 13: DESCRIPTION OF A MUTILATED LOWER JAW OF THE MEGALONYX JEFFERSONII.
By Richard Owen
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About This Book

The work offers a geological introduction followed by systematic, anatomical descriptions of fossil mammals collected on the voyage, with emphasis on South American extinct forms. It details crania, jaws, teeth, vertebrae, limb bones, and tesselated armour for taxa such as Toxodon, Macrauchenia, Glossotherium, Mylodon, Scelidotherium, Megalonyx, and Megatherium, illustrated by numerous plates. Comparative morphology and measurements underpin taxonomic identifications and interpretations of structural relationships, and a brief discussion addresses the geological contemporaneity of the described extinct mammals.

DESCRIPTION OF A MUTILATED LOWER JAW OF THE
MEGALONYX JEFFERSONII.

In the preceding section an astralagus was described, which was regarded as belonging possibly to the same Edentate species as the jaw figured and described, p. 69, Pl. XVIII. and XIX., under the name of Mylodon Darwinii; but the same correspondence,—that of relative size,—renders it equally possible that this astragalus may belong to the species of Megalonyx to which the lower jaw now under consideration appertains. There could be no doubt, from its structure, that it was the astragalus of a gigantic species of the order Bruta, and of the Megatherioid family, and more nearly allied to the Megathere than is the Scelidothere, but sufficiently distinct from both.

The lower jaw, figured in Pl. XXIX., is the only fossil brought home by Mr. Darwin that could be confidently referred to the genus Megalonyx; but the form of the tooth in place on the right side of the jaw fully justifies this determination. The jaw itself is deeply and firmly imbedded in the matrix, so that only the upper or alveolar border is visible. The coronoid and condyloid processes are broken away, and the texture of the remaining part of the jaw was too friable, and adhered too firmly to the surrounding matrix to admit of more of its form being ascertained than is figured.

There were four molars on each side of this jaw; the large oblique perforation near the fractured symphysis is the anterior extremity of the wide dental canal. The forms of the alveoli are best preserved in the right ramus: the first is the smallest, and seems to have contained a tooth, of which the transverse section must have been simply elliptical: the second tooth is likewise laterally compressed, but the transverse section is ovate, the great end being turned forwards: the third socket presents a corresponding form, but a larger size: the fourth socket is too much mutilated to allow of a correct opinion being formed as to the shape of the tooth which it once contained. The natural size of the tooth in situ, and of the adjoining socket, is given in Pl. XXIX., fig. 2. The difference of form which the jaw of the Megalonyx presents, as compared with that of the Mylodon, especially in the greater recedence of the two horizontal rami from each other, will be appreciated by comparing Pl. XVIII. with Pl. XXIX.