Nov.] AN EXISTING <: ROUND-SLOTH IN PATAGONIA 325 



which was believed to be quite extinct, a gravigrade edentate related 

 to Mylodon and Pseudolestodon. 



The gravigrade edentates are reckoned among the oldest mammals 

 which appeared upon the earth. The most ancient traces of them 

 have been observed below the Guaranian Formation, with gigantic 

 Dinosaurs, in the variegated sandstones of Patagonia, which are 

 referred to the Lower Cretaceous. They become more numerous in 

 the PyrotheHum beds of the Guaranian, develop gradually, and 

 attain their greatest diversity during the Upper Eocene (Santa Cruz 

 Formation). Thenceforward their variety decreases, but their size 

 gradually increases, until in the Pampean they are represented by 

 a certain number of gigantic forms, such as Megatherium, Lestodon, 

 Mylodon, etc. Rare fragments in a bad state of preservation have 

 been found even in the Post-Pampean deposits, but no one had 

 supposed that they still had living representatives. 



Some of the Pampean genera show a very curious character : 

 the body was protected on all sides by an incredible number of small 

 irregular ossicles, which it is supposed were developed in the thick- 

 ness of the skin, and thus became covered with a horny or scaly 

 epidermis. The genera showing this peculiarity are Mylodon, Pseudo- 

 lestodon, and Glossotln riztm. The other genera, such as Megatherium, 

 Lestodon, and Scelidotherium, do not show any trace of it. Besides 

 in the Pampean Formation these ossicles are met with in the Arau- 

 canian Formation of Monte Hermoso and Catamarca, and also in the 

 Kntrerios Formation ; but no trace of them has been found in the 

 Santacruzian, where the gravigrade edentates are so abundant, or in 

 the earlier formations. We conclude from this that the character in 

 question is not primitive, but acquired secondarily at a relatively 

 modern period. 



These ossicles, comparable to large coffee berries, differ slightly 

 in shape and size according to the genera. In Glossotkerium they 

 are large and flattened ; in Mylodon they are smaller, irregular, 

 elliptical, trapezoidal, or rhomboidal, with one side more convex or 

 keeled, their diameter varying from one to two centimetres, though 

 sometimes less. Their surface, more especially on the flattest side, 

 shows some tiny depressions and perforations, and reticular tracery 

 well seen under the magnifying glass. Their aspect is so character- 

 istic that when one has once seen them they are recognised immedi- 

 ately without any danger of being mistaken. 



Lately, several little ossicles have been brought to me from 

 Southern Patagonia, and I have been asked to what animal they 

 could belong. What was my surprise on seeing in my hand these 

 ossicles in a fresh state, and, notwithstanding that, absolutely simi- 

 lar to the fossil dermal ossicles of the genus Mylodon, except only 

 that they are of smaller size, varying from 9 to 13 or 14 mm. 



