278 MAMMALIA. 



crown, which disappears early in life through wear and leaves 

 the dentine exposed. In the earliest known forms the teeth 

 are rooted and more or less completely enamelled ; in the later 

 forms the incisors gradually diminish, and all the teeth become 

 hypsodont, rootless, and of persistent growth, while the enamel 

 is limited to narrow vertical bands. In one tribe the facial 

 part of the skull is short, and the only known fore limb is 

 remarkably similar to that of the extinct ground-sloths, 

 Megatherium and its allies. In another tribe the skull is 

 more elongated, and the few known parts of the skeleton 

 exhibit much resemblance to the corresponding bones of the 

 modern armadillos. To the first tribe are referred Hemiganus 

 and Psittacotherium from the Puerco Eocene, Ectoganus and 

 Calamodon from the Wasatch Eocene, and Stylinodon from the 

 Bridger Eocene. The Puerco Eocene genus Conoryctes repre- 

 sents the second tribe or family. Some teeth and a portion of 

 mandible from the Upper Eocene of Switzerland have also 

 been referred to Calamodon with much probability of correct- 

 ness. 



The earliest Tertiary Edentata hitherto discovered in South 

 America, namely, those from the Pyrotherium Formation of 

 Patagonia, seem to be a little more specialized than the North 

 American forms just mentioned ; but one armadillo (Peltephilus) 

 at least has a series of front teeth, and several genera exhibit 

 traces of enamel on the teeth. The armoured type is already 

 clearly differentiated from the sloth and anteater type, but a 

 certain amount of evolution can be observed as the various 

 genera are traced through the successive Tertiary formations 

 of the Argentine Republic. 



The modern sloths, Brady podidae. and anteaters, Myrme- 

 cophagidae, are practically unknown among fossils ; but an 

 important extinct family, that of the ground-sloths or Mega- 

 theriidae, is of special interest as combining in the skeleton 

 the characters of the two existing families just mentioned. 

 These animals exhibit the head and teeth of a sloth, associated 

 with the vertebrae, limbs, and tail of the anteater. Some of 

 them, like the typical genus Megatherium, attain a gigantic 

 size, at least as large as an elephant ; and their remains occur 



