OCR Text |
Show 1899.] NEOMYLODON LISTAI FROM PATAGONIA. 153 this extinct genus may have been uniform throughout, only differing in size and sparseness or compactness-in which case Dr. Ameghino is justified in proposing to recognize a distinct genus, Neomylodon. To decide between these two possibilities, it is necessary to Avait for additional information concerning the anterior dorsal armour of Mylodon as precise as that published by Burmeister in reference to the lumbar shield. Meanwhile it must suffice to compare the microscopical structure of the ossicles from the new skin with that of the small sculptured tubercles of undoubted Mylodon represented in PI. X V . figs. 4-6. Part of a horizontal section of one of these fossil ossicles is shown enlarged about 40 times in PI. X V . fig. 8. It must be remembered that the specimen has been buried in the Pampa Formation for a long period, and that the oxides of iron and manganese have infiltrated the- margin of the bone, rendering the structure of its outer border more conspicuous than that of its central portion. It must also be noted that some of the manganese has assumed its familiar " dendritic" aspect, in this respect presenting appearances not due to original structure. The calcified interlacing fibres of connective tissue are as abundant here as in the ossicle of the so-called Neomylodon ; but in a very wide peripheral area they exhibit a marked radial disposition, nearly everywhere extending in bundles at right angles to the border. Rather large vascular canals, infiltrated Avith the oxides of iron and manganese, are observed in places, often bifurcated and usually bordered by a transparent zone free from the connective-tissue fibres. Well-developed bone-lacuna) are very abundant, many exhibiting short branching canaliculi (PI. X V . fig. 8 a), and most of the others very irregular in shape, evidently furnished Avith canaliculi Avhich cannot be seen from lack of infiltration. The lacuna) are never much elongated, and are not arranged in distinctly differentiated Haversian systems in any part of the section ; while the only regular disposition of the bony lamina) is traceable near the circumference, where the lacunae are frequently arranged or clustered iu parallel zones concentric with the border. A vertical section of one of the same specimens shoAvs the connective-tissue fibres radiating outwards towards the lateral margins, but not directly towards the upper sculptured face. There are no bony lamina) clearly parallel with the latter face, and at least one vascular canal in transverse section seems to be the centre of a Haversian system. The histological structure of the ossicles in the skin noAv under consideration thus resembles that of the sculptured tubercles of Mylodon in all essential features, but differs in two noteworthy respects. In the ossicles of the so-called Neomylodon, as already described, the fibres of connective tissue do not exhibit much definite radiation towards the lateral margin; Avhile the bony tissue at most points is disposed in definite Haversian systems. There is thus enough discrepancy to justify the suspicion that the new7 and the old specimens do not belong to the same animal. |