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Show 498 MR. R. LYDEKKER ON SCELIDOTHERIUM. [Nov. 16, genus occupying in some respects an intermediate position between Megatherium and Mylodon, but also showing evidence of a still more widely extended affinity. The dentition is decidedly nearest to that of Mylodon, while the hind foot approximates to that of Megatherium. The crania of species like 8. chiliense and S. bravardi are those least removed from the Megatherium type, and it is these species which come nearest to that genus in the structure of the astragalus. All those forms in which the pes is known exhibit the anchylosis of the first and second phalangeals of the third digit, and the large claw of the same, which are such characteristic features of the type genus of the family. The peculiar Mylodon darwini (generically separated by Reinhardt under the name of Grypotherium) is the form by which Scelidotherium is connected by cranial characters with Mylodon ; and the connection is so close that it becomes somewhat difficult to give a clear differential diagnosis. In its extremely elongated facial region and peculiar astragalus, S. leptocephalum is the species departing most widely from the Meya-therium type, and it is probable, from the structure of the last-named bone, that in this animal the pes was not everted as it is in Meya-therium. In both its peculiar features S. leptocephalum makes such a very marked approach to the Myrmecophayido?, that it is quite easy to imagine how that family may have taken origin from some member of the Megatheriida*; while the remarkable resemblance in dental characters existing between those members of the genus Mylodon which have been separated by some writers under the names of Pseudolestodon and Leslodon and the Bradypodidce suggests that the modern arboreal Sloths may also originally have sprung from some early member of the same great family of Ground- Sloths. EXPLANATION OF T H E PLATES. PLATE XLVI. Scelidotherium leptocephalum, Owen. Frontal aspect of the cranium ; from the Argentine Republic. British Museum, No. 37308. The occiput has been restored from another specimen. J. la, lachrymal; na, nasal; mx, maxilla. PLATE XLVII. Scelidotherium bravardi, Lydekker. Frontal aspect of the imperfect cranium ; from the Argentine Republic. British Museum, No. 37626. i. Letters as in Plate XLVI. PLATE XLVIII. Scelidotherium chiliense, Lydekker. Frontal aspect of the cranium, British Museum, No. M. 2819. J. Letters as in Plate XLVI. PLATE XLIX. Fig. 1. Scelidotherium bravardi, Lydekker. The mandible associated with the cranium figured in Plate XLVII. British Museum, No. 37649. }. 2. Scelidotherium chiliense, Lydekker. The anterior part of the mandible ; from Chili. British Museum, No. M. 2821. £. 3. Scelidotherium leptocephalum, Owen. The left astragalus; from Patagonia. Mus. Boy. Coll. Surgeons, No. 3520. £. a, external trochlear ridge for tibia ; b, internal tuberosity. 4. (?) Scelidotherium bravardi, Lydekker. The left astragalus ; from Brazil. British Museum, No. 18620 k. £. Letters as in fig. 3. |
