1899.] TfEOMTLODON LISTAI FROM PATAGONIA. 153 



this extinct genus may have been uniform throughout, only 

 differing in size and sparseuess 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 wait 

 for additional information concerning the anterior dorsal armour 

 of Mi/Iodon 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 PL XV. figs. 4-6. Part of a horizontal section of 

 one of these fossil ossicles is shown enlarged about 40 times in 

 PI. XV. 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 kindles at right angles to the border. Eather large vascular 

 canals, infiltrated with the oxides of iron and manganese, are ob- 

 served in places, often bifurcated and usually bordered by a trans- 

 parent zone free from the connective-tissue fibres. Well- developed 

 bone-lacunsB are very abundant, many exhibiting short branching 

 canaUculi (PI. XV. fig. 8 a), and most of the others very irregular 

 in shape, evidently furnished with canaliculi which 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 laminae is traceable near the circumference, where the 

 lacunae are frequently arranged or clustered in parallel zones 

 concentric with the border. A vertical section of one of the 

 same specimens shows the connective-tissue fibres radiating out- 

 wards towards the lateral margins, but not directly towards the 

 upper sculptured face. There are no bony laminae 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 now 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; while the bony 

 tissue at most points is disposed in definite Haversian systems. 

 There is thus enough discrepancy to justify the suspicion that 

 the new and the old specimens do not belong to the same animal. 



