APPENDIX A 311 



armour sucli as is exhibited both by the existing Armadillos and the 

 extinct Glyptodonts. There are, then, two possibilities. Either the 

 dermal armour oi Mylodon varied in different parts of the body, being 

 sculptured and covered only by epidermis in the lumbar region, while 

 less developed, not sculptured but completely buried in the dermis in the 

 comparatively flexible neck and shoulder region — in whicli case Dr. Moreno 

 may be correct in referring the problematical specimen to Mylodon ; or 

 the dermal ossicles of this extinct genus may have been uniform through- 

 out, only differing in size and sparseness or compactness — in which case 

 Dr. Ameghino is justified in proposing to recognise a distinct genus, 

 Neoinylodon. 



To decide between these two possibilities, it is necessary to wait 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. 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 

 Neoniylodon ; but in a very wide peripheral area they exhibit a marked 

 radial di.-p^sition, neariy every wiicc extending in bundles at right angles 

 to the border. Rather large vascular canals, infiltrated with the oxides 

 of iron and manganese, are observed in places, often bifurcated and usually 

 bordered by a transparent zone free from the connective-ti.-sue fibres. 

 Well-developed bone-lacunte are very abundant, many exhibiting short 

 branching canaliculi, and most of the others very irregular in shape, 

 evidently furnished with canaliculi which cannot be seen from lack of 

 infiltration. The lacunae are never much elongated, and are not arranged 

 in distinctly differentiated Haversian systems in any jjart 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 bordei. A vertical >ection 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 laminio clearly parallel with the 

 latter face, and at least one vascular canal in tran>verse section se«Mns 10 

 be the centre of a Haversian system. 



The histological structure of the ossicles in the skin now under 



